Production process for alkali metal-sulfur batteries having high volumetric and gravimetric energy densities

ABSTRACT

Process for producing an alkali metal-sulfur battery, comprising: (a) Preparing a first conductive porous structure; (b) Preparing a second conductive porous structure; (c) Injecting or impregnating a first suspension into pores of the first conductive porous structure to form an anode electrode, wherein the first suspension contains an anode active material, an optional conductive additive, and a first electrolyte; (d) Injecting or impregnating a second suspension into pores of the second conductive porous structure to form a cathode electrode, wherein the second suspension contains a cathode active material (selected from sulfur, lithium polysulfide, sodium polysulfide, sulfur-polymer composite, organo-sulfide, sulfur-carbon composite, sulfur-graphene composite, or a combination thereof), an optional conductive additive, and a second electrolyte; and (e) Assembling the anode electrode, a separator, and a cathode electrode into the battery.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/998,513 filed Jan. 15, 2016, the contents of which arehereby incorporated by reference for all purposes.

FIELD OF THE INVENTION

This invention is directed at a secondary (rechargeable) lithium-sulfurbattery (including Li—S and Li ion-S cells) or sodium-sulfur battery(including Na—S and Na ion-S cells) having a high volumetric energydensity and a high gravimetric energy density.

BACKGROUND

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (includingLi-sulfur and Li metal-air batteries) are considered promising powersources for electric vehicle (EV), hybrid electric vehicle (HEV), andportable electronic devices, such as lap-top computers and mobilephones. Lithium as a metal element has the highest capacity (3,861mAh/g) compared to any other metal or metal-intercalated compound as ananode active material (except Li_(4.4)Si, which has a specific capacityof 4,200 mAh/g). Hence, in general, Li metal batteries have asignificantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having relatively high specific capacities, suchas TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathode active materials,which were coupled with a lithium metal anode. When the battery wasdischarged, lithium ions were transferred from the lithium metal anodethrough the electrolyte to the cathode, and the cathode becamelithiated. Unfortunately, upon repeated charges/discharges, the lithiummetal resulted in the formation of dendrites at the anode thatultimately grew to penetrate through the separator, causing internalshorting and explosion. As a result of a series of accidents associatedwith this problem, the production of these types of secondary batterieswas stopped in the early 1990's, giving ways to lithium-ion batteries.

In lithium-ion batteries, pure lithium metal sheet or film was replacedby carbonaceous materials as the anode. The carbonaceous materialabsorbs lithium (through intercalation of lithium ions or atoms betweengraphene planes, for instance) and desorbs lithium ions during there-charge and discharge phases, respectively, of the lithium ion batteryoperation. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of lithium transition-metal oxide or phosphate basedcathode active material is typically in the range of 140-170 mAh/g. As aresult, the specific energy of commercially available Li-ion cells istypically in the range of 120-250 Wh/kg, most typically 150-220 Wh/kg.These specific energy values are two to three times lower than whatwould be required for battery-powered electric vehicles to be widelyaccepted.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials that provide a rechargeablebattery with a significantly higher specific energy, higher energydensity, higher rate capability, long cycle life, and safety. One of themost promising energy storage devices is the lithium-sulfur (Li—S) cellsince the theoretical capacity of Li is 3,861 mAh/g and that of S is1,675 mAh/g. In its simplest form, a Li—S cell consists of elementalsulfur as the positive electrode and lithium as the negative electrode.The lithium-sulfur cell operates with a redox couple, described by thereaction S₈+16L⇄8Li₂S that lies near 2.2 V with respect to Li⁺/Li^(o).This electrochemical potential is approximately ⅔ of that exhibited byconventional positive electrodes (e.g. LiMnO₄) in a conventionallithium-ion battery. However, this shortcoming is offset by the veryhigh theoretical capacities of both Li and S. Thus, compared withconventional intercalation-based Li-ion batteries, Li—S cells have theopportunity to provide a significantly higher energy density (a productof capacity and voltage). Assuming complete reaction to Li₂S, energydensities values can approach 2,500 Wh/kg and 2,800 Wh/L, respectively,based on the combined Li and S weight or volume. If based on the totalcell weight or volume, the energy densities can reach approximately1,000 Wh/kg and 1,100 Wh/L, respectively. However, the current Li-sulfurcells reported by industry leaders in sulfur cathode technology have amaximum cell specific energy of 250-400 Wh/kg and 500-650 Wh/L (based onthe total cell weight or volume), which are far below what is possible.

In summary, despite its considerable advantages, the Li—S cell isplagued with several major technical problems that have thus farhindered its widespread commercialization:

-   (1) Conventional lithium metal cells still have dendrite formation    and related internal shorting issues.-   (2) Sulfur or sulfur-containing organic compounds are highly    insulating, both electrically and ionically. To enable a reversible    electrochemical reaction at high current densities or    charge/discharge rates, the sulfur must maintain intimate contact    with an electrically conductive additive. Various carbon-sulfur    composites have been utilized for this purpose, but only with    limited success owing to the limited scale of the contact area.    Typical reported capacities are between 300 and 550 mAh/g (based on    the cathode carbon-sulfur composite weight) at moderate rates.-   (3) The cell tends to exhibit significant capacity decay during    discharge-charge cycling. This is mainly due to the high solubility    of the lithium polysulfide anions formed as reaction intermediates    during both discharge and charge processes in the polar organic    solvents used in electrolytes. During cycling, the lithium    polysulfide anions can migrate through the separator to the Li    negative electrode whereupon they are reduced to solid precipitates    (Li₂S₂ and/or Li₂S), causing active mass loss. In addition, the    solid product that precipitates on the surface of the positive    electrode during discharge becomes electrochemically irreversible,    which also contributes to active mass loss.-   (4) More generally speaking, a significant drawback with cells    containing cathodes comprising elemental sulfur, organosulfur and    carbon-sulfur materials relates to the dissolution and excessive    out-diffusion of soluble sulfides, polysulfides, organo-sulfides,    carbon-sulfides and/or carbon-polysulfides (hereinafter referred to    as anionic reduction products) from the cathode into the rest of the    cell. This phenomenon is commonly referred to as the Shuttle Effect.    This process leads to several problems: high self-discharge rates,    loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact to active    cell components, fouling of the anode surface giving rise to    malfunction of the anode, and clogging of the pores in the cell    membrane separator which leads to loss of ion transport and large    increases in internal resistance in the cell.

In response to these challenges, new electrolytes, protective films forthe lithium anode, and solid electrolytes have been developed. Someinteresting cathode developments have been reported recently to containlithium polysulfides; but, their performance still fall short of what isrequired for practical applications.

For instance, Ji, et al reported that cathodes based on nanostructuredsulfur/meso-porous carbon materials could overcome these challenges to alarge degree, and exhibit stable, high, reversible capacities with goodrate properties and cycling efficiency [Xiulei Ji, Kyu Tae Lee, & LindaF. Nazar, “A highly ordered nanostructured carbon-sulphur cathode forlithium-sulphur batteries,” Nature Materials 8, 500-506 (2009)].However, the fabrication of the proposed highly ordered meso-porouscarbon structure requires a tedious and expensive template-assistedprocess. It is also challenging to load a large proportion of sulfurinto these meso-scaled pores using a physical vapor deposition orsolution precipitation process.

Zhang, et al. (US Pub. No. 2014/0234702; Aug. 21, 2014) makes use of achemical reaction method of depositing S particles on surfaces ofisolated graphene oxide (GO) sheets. But, this method is incapable ofcreating a large proportion of S particles on GO surfaces (i.e.typically <66% of S in the GO-S nanocomposite composition). Theresulting Li—S cells also exhibit poor rate capability; e.g. thespecific capacity of 1,100 mAh/g (based on S weight) at 0.02 C rate isreduced to <450 mAh/g at 1.0 C rate. It may be noted that the highestachievable specific capacity of 1,100 mAh/g represents a sulfurutilization efficiency of only 1,100/1,675=65.7% even at such a lowcharge/discharge rate (0.02 C means completing the charge or dischargeprocess in 1/0.02=50 hours; 1 C=1 hour, 2 C=½ hours, and 3 C=⅓ hours,etc.) Further, such a S-GO nanocomposite cathode-based Li—S cellexhibits very poor cycle life, with the capacity typically dropping toless than 60% of its original capacity in less than 40 charge/dischargecycles. Such a short cycle life makes this Li—S cell not useful for anypractical application. Another chemical reaction method of depositing Sparticles on graphene oxide surfaces is disclosed by Wang, et al. (USPub. No. 2013/0171339; Jul. 4, 2013). This Li—S cell still suffers fromthe same problems.

A solution precipitation method was disclosed by Liu, et al. (US Pub.No. 2012/0088154; Apr. 12, 2012) to prepare graphene-sulfurnanocomposites (having sulfur particles adsorbed on GO surfaces) for useas the cathode material in a Li—S cell. The method entails mixing GOsheets and S in a solvent (CS₂) to form a suspension. The solvent isthen evaporated to yield a solid nanocomposite, which is then ground toyield nanocomposite powder having primary sulfur particles with anaverage diameter less than approximately 50 nm. Unfortunately, thismethod does not appear to be capable of producing S particles less than40 nm. The resulting Li—S cell exhibits very poor cycle life (a 50%decay in capacity after only 50 cycles). Even when these nanocompositeparticles are encapsulated in a polymer, the Li—S cell retains less than80% of its original capacity after 100 cycles. The cell also exhibits apoor rate capability (specific capacity of 1,050 mAh/g(S wt.) at 0.1 Crate, dropped to <580 mAh/g at 1.0 C rate). Again, this implies that alarge proportion of S did not contribute to the lithium storage,resulting in a low S utilization efficiency.

Despite the various approaches proposed for the fabrication of highenergy density Li—S cells, there remains a need for cathode materialsand production processes that improve the utilization of electro-activecathode materials (S utilization efficiency), and provide rechargeableLi—S cells with high capacities over a large number of cycles.

Most significantly, lithium metal (including pure lithium, lithiumalloys of high lithium content with other metal elements, orlithium-containing compounds with a high lithium content; e.g. >80% orpreferably >90% by weight Li) still provides the highest anode specificcapacity as compared to essentially all other anode active materials(except pure silicon, but silicon has pulverization issues). Lithiummetal would be an ideal anode material in a lithium-sulfur secondarybattery if dendrite related issues could be addressed.

Sodium metal (Na) and potassium metal (K) have similar chemicalcharacteristics to Li and the sulfur cathode in room temperaturesodium-sulfur cells (RT Na—S batteries) or potassium-sulfur cells (K—S)face the same issues observed in Li—S batteries, such as: (i) low activematerial utilization rate, (ii) poor cycle life, and (iii) low Coulombicefficiency. Again, these drawbacks arise mainly from insulating natureof S, dissolution of S and Na or K polysulfide intermediates in liquidelectrolytes (and related Shuttle effect), and large volume changeduring charge/discharge.

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the sulfur or lithium polysulfideweight alone (not the total cathode composite weight), but unfortunatelya large proportion of non-active materials (those not capable of storinglithium, such as conductive additive and binder) is typically used intheir Li—S cells. For practical use purposes, it is more meaningful touse the cathode composite weight-based capacity value.

Low-capacity anode or cathode active materials are not the only problemassociated with the lithium-sulfur or sodium-sulfur battery. There areserious design and manufacturing issues that the battery industry doesnot seem to be aware of, or has largely ignored. For instance, despitethe seemingly high gravimetric capacities at the electrode level (basedon the anode or cathode active material weight alone) as frequentlyclaimed in open literature and patent documents, these electrodesunfortunately fail to provide batteries with high capacities at thebattery cell or pack level (based on the total battery cell weight orpack weight). This is due to the notion that, in these reports, theactual active material mass loadings of the electrodes are too low. Inmost cases, the active material mass loadings of the anode (arealdensity) is significantly lower than 15 mg/cm² and mostly <8 mg/cm²(areal density=the amount of active materials per electrodecross-sectional area along the electrode thickness direction). Thecathode active material amount is typically 1.5-2.5 times higher thanthe anode active material amount in a cell. As a result, the weightproportion of the anode active material (e.g. carbon) in a Na ion-sulfuror Li ion-sulfur battery cell is typically from 15% to 20%, and that ofthe cathode active material from 20% to 35% (mostly <30%). The weightfraction of the cathode and anode active materials combined is typicallyfrom 35% to 50% of the cell weight.

The low active material mass loading is primarily due to the inabilityto obtain thicker electrodes (thicker than 100-200 μm) using theconventional slurry coating procedure. This is not a trivial task as onemight think, and in reality the electrode thickness is not a designparameter that can be arbitrarily and freely varied for the purpose ofoptimizing the cell performance. Contrarily, thicker samples tend tobecome extremely brittle or of poor structural integrity and would alsorequire the use of large amounts of binder resin. Due to the low-meltingand soft characteristics of sulfur, it has been practically impossibleto produce a sulfur cathode thicker than 100m. Furthermore, in a realbattery manufacturing facility, a coated electrode thicker than 150 μmwould require a heating zone as long as 100 meters to thoroughly dry thecoated slurry.

This would significantly increase the equipment cost and reduce theproduction throughput. The low areal densities and low volume densities(related to thin electrodes and poor packing density) result in arelatively low volumetric capacity and low volumetric energy density ofthe battery cells.

With the growing demand for more compact and portable energy storagesystems, there is keen interest to increase the utilization of thevolume of the batteries. Novel electrode materials and designs thatenable high volumetric capacities and high mass loadings are essentialto achieving improved cell volumetric capacities and energy densities.

Thus, an object of the present invention is to provide a rechargeablealkali metal-sulfur cell based on rational materials and battery designsthat overcome or significantly reduce the following issues commonlyassociated with conventional Li—S and Na—S cells: (a) dendrite formation(internal shorting); (b) extremely low electric and ionic conductivitiesof sulfur, requiring large proportion (typically 30-55%) of non-activeconductive fillers and having significant proportion of non-accessibleor non-reachable sulfur or alkali metal polysulfides); (c) dissolutionof S and alkali metal polysulfide in electrolyte and migration ofpolysulfides from the cathode to the anode (which irreversibly reactwith Li or Na metal at the anode), resulting in active material loss andcapacity decay (the shuttle effect); (d) short cycle life; and (e) lowactive mass loading in both the anode and the cathode.

A specific object of the present invention is to provide a rechargeablealkali metal-sulfur battery (e.g. mainly Li—S and room temperature Na—Sbattery) that exhibits an exceptionally high specific energy or highenergy density. One particular technical goal of the present inventionis to provide an alkali metal-sulfur or alkali ion-sulfur cell with acell specific energy greater than 400 Wh/Kg, preferably greater than 500Wh/Kg, more preferably greater than 600 Wh/Kg, and most preferablygreater than 700 Wh/kg (all based on the total cell weight). Preferably,the volumetric energy density is greater than 600 Wh/L, furtherpreferably greater than 800 Wh/L, and most preferably greater than 1,000Wh/L.

Another object of the present invention is to provide an alkalimetal-sulfur cell that exhibits a high cathode specific capacity, higherthan 1,200 mAh/g based on the sulfur weight, or higher than 1,000 mAh/gbased on the cathode composite weight (including sulfur, conductingadditive or substrate, and binder weights combined, but excluding theweight of cathode current collector). The specific capacity ispreferably higher than 1,400 mAh/g based on the sulfur weight alone orhigher than 1,200 mAh/g based on the cathode composite weight. This mustbe accompanied by a high specific energy, good resistance to dendriteformation, and a long and stable cycle life.

SUMMARY OF THE INVENTION

The present invention provides an alkali metal-sulfur battery that has ahigh cathode active material mass loading, a thick cathode, a highsulfur cathode specific capacity, an exceptionally low overhead weightand volume (relative to the active material mass and volume), a highgravimetric energy density, and a high volumetric energy density thathave never been previously achieved. The invention includes both thelithium metal-sulfur and room temperature sodium metal-sulfur cells.

The lithium-sulfur batteries include (a) the lithium metal-sulfur (Li—Scell) that uses a Li metal or Li metal alloy (e.g. Li foil) as the mainanode active material and sulfur, polysulfide, and/or sulfur-carboncompound as the main cathode active material and (b) the lithiumion-sulfur cell that makes use of a lithium intercalation compound (e.g.graphite and Si) as the main anode active material and sulfur,polysulfide, and/or sulfur-carbon compound as the main cathode activematerial. The sodium-sulfur batteries include (a) the sodiummetal-sulfur (Na—S cell) that uses a Na metal or Na metal alloy (e.g. Nafoil) as the main anode active material and sulfur, polysulfide, and/orsulfur-carbon compound as the main cathode active material and (b) thesodium ion-sulfur cell that makes use of a sodium intercalation compound(e.g. hard carbon particles and Sn) as the main anode active materialand sulfur, polysulfide, and/or sulfur-carbon compound as the maincathode active material.

In one embodiment, the presently invented battery comprises:

-   -   (a) an anode having (i) an anode active material slurry (or        suspension) comprising an anode active material and an optional        conductive additive dispersed in a first electrolyte and (ii) a        conductive porous structure acting as a 3D anode current        collector wherein the conductive porous structure has at least        70% by volume of pores and wherein the anode active material        slurry is disposed in pores of the anode conductive porous        structure (the terms “anode conductive porous structure” and “3D        anode current collector” are herein used interchangeably);    -   (b) a cathode having (i) a cathode active material slurry        comprising a cathode active material and an optional conductive        additive dispersed in a second electrolyte (preferably a liquid        or gel electrolyte), the same as or different than the first        liquid or gel electrolyte, and (ii) a conductive porous        structure acting as a 3D cathode current collector wherein the        conductive porous structure has at least 70% by volume of pores        and wherein the cathode active material slurry is disposed in        pores of the cathode conductive porous structure (the terms        “cathode conductive porous structure” and “3D cathode current        collector” are herein used interchangeably);

The cathode active material is selected from sulfur bonded to pore wallsof the cathode current collector, sulfur bonded to or confined by acarbon or graphite material, sulfur bonded to or confined by a polymer,sulfur-carbon compound, metal sulfide M_(x)S_(y), wherein x is aninteger from 1 to 3 and y is an integer from 1 to 10, and M is a metalelement selected from Li, Na, K, Mg, Ca, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof;and

-   -   (c) a separator disposed between the anode and the cathode.    -   In this battery, the anode thickness-to-anode current collector        thickness ratio is from 0.8/1 to 1/0.8 and/or the cathode        thickness-to-cathode current collector thickness ratio is from        0.8/1 to 1/0.8. The 3D porous anode current collector or cathode        current collector has a thickness no less than 200 μm, the        cathode active material constitutes an electrode active material        loading greater than 10 mg/cm² (preferably >15 mg/cm², further        preferably >20 mg/cm² , and most preferably >30 mg/cm²) and/or        the anode active material and the cathode active material        combined exceeds 40% by weight of the total battery cell weight.

This alkali metal-sulfur battery may be produced by a process (as anexample), comprising: (a) Preparing a first conductive porous structureor first conductive foam layer; (b) Preparing a second conductive porousstructure or second conductive foam layer; (c) Injecting or impregnatinga first suspension into pores of the first conductive porous structureto form an anode electrode, wherein the first suspension contains ananode active material, an optional conductive additive, and a firstliquid or gel electrolyte and wherein the anode electrode and the firstconductive porous structure are similar or comparable in shape anddimensions; (d) Injecting or impregnating a second suspension into poresof the second conductive porous structure to form a cathode electrode,wherein the second suspension contains a cathode active material, anoptional conductive additive, and a second liquid or gel electrolyte,wherein the cathode active material is selected from sulfur, lithiumpolysulfide, sodium polysulfide, sulfur-polymer composite,organo-sulfide, sulfur-carbon composite, sulfur-graphene composite, or acombination thereof; and wherein the cathode electrode and the secondconductive porous structure are similar or comparable in shape anddimensions; and (e) Assembling the anode electrode, a separator, and acathode electrode into said alkali metal-sulfur battery.

In some embodiments, step (a), (b), (c), (d), and (e) are conducted inthe following sequence:

-   -   (A) Preparing the first suspension and the second suspension;    -   (B) Assembling a porous cell framework composed of the first        conductive porous structure as a porous anode current collector,        the second conductive porous structure as a porous cathode        current collector, and a porous separator disposed between the        porous anode current collector and the porous cathode current        collector; wherein the porous anode current collector and/or the        porous cathode current collector has a thickness no less than        200 μm and at least 70% by volume of pores; and    -   (C) Injecting the first suspension into pores of the anode        current collector to form the anode electrode and injecting the        second suspension into pores of the cathode current collector to        form the cathode electrode to an extent that the anode active        material has a material mass loading no less than 20 mg/cm² in        the anode electrode or the cathode active material has a        material mass loading no less than 10 mg/cm² (preferably >15        mg/cm², further preferably>20 mg/cm² , and most preferably >30        mg/cm²) in the cathode electrode; wherein the anode current        collector, the separator, and the cathode current collector are        assembled in a protective housing before or after the injection        or impregnation of first suspension and/or the injection or        impregnation of second suspension.

In a preferred embodiment, the process comprises:

-   -   A) Assembling a porous cell framework composed of a first        conductive porous structure (e.g. a conductive foam or an        interconnected 3D network of electron-conducting paths) as a 3D        anode current collector, a second conductive porous structure        (e.g. a conductive foam) as a 3D cathode current collector, and        a porous separator disposed between the first and second        conductive porous structure; wherein the first and/or second        conductive foam structure has a thickness no less than 200 μm        (preferably greater than 300 μm, more preferably greater than        400 μm, further preferably greater than 500 μm, and most        preferably greater than 600 μm) and at least 70% by volume of        pores (preferably at least 80% porosity, more preferably at        least 90%, and most preferably at least 95%; these pore volumes        referring to amounts of pores prior to being impregnated with an        electrode active material slurry or suspension);    -   B) Preparing a first suspension of an anode active material and        an optional conductive additive dispersed in a first liquid or        gel electrolyte and a second suspension of a cathode active        material and an optional conductive additive dispersed in a        second liquid or gel electrolyte; and    -   C) Impregnating the pores of the first conductive porous        structure with the first suspension (e.g. injecting the first        suspension into pores of the first conductive porous structure)        to form an anode and impregnating the pores of the second        conductive porous structure with the second suspension (e.g.        injecting the second suspension into pores of the second        conductive foam structure) to form a cathode to the extent that        preferably the anode active material has a material mass loading        no less than 20 mg/cm² in the anode or the cathode active        material has a material mass loading no less than 10 mg/cm²        (preferably >15 mg/cm², further preferably >20 mg/cm² , and most        preferably >30 mg/cm²) in the cathode.        The anode current collector, the separator, and the cathode        current collector are assembled in a protective housing before,        during or after the injecting (or impregnation) of the first        suspension and/or the injecting (or impregnation) of the second        suspension.

In certain embodiments, the above step (a), (b), (c), (d), and (e) areconducted in the following sequence:

-   -   (A) Preparing one or a plurality of electrically conductive        porous layers, one or a plurality of wet anode layers of the        first suspension, and one or a plurality of wet cathode layers        of the second suspension, wherein the conductive porous layers        contain interconnected conductive pathways and at least 70% by        volume of pores;    -   (B) Stacking and consolidating a desired number of the porous        layers and a desired number of the wet anode layers in a        sequence to form said anode electrode having a thickness no less        than 200 μm;    -   (C) Placing the porous separator layer in contact with the anode        electrode;    -   (D) Stacking and consolidating a desired number of the porous        layers and a desired number of the wet cathode layers in a        sequence to form the cathode electrode in contact with the        porous separator, wherein the cathode electrode has a thickness        no less than 200 μm; and    -   (E) Assembling and sealing the anode electrode, porous        separator, and cathode electrode in a housing to produce the        alkali metal-sulfur battery;        wherein the anode active material has a material mass loading no        less than 20 mg/cm² in the anode electrode and/or said cathode        active material has a material mass loading no less than 15        mg/cm² in the cathode electrode.

Another embodiment of the present invention is an alkali metal-sulfurbattery, comprising:

-   -   (a) an anode having an anode active material coated on or in        physical contact with an anode current collector wherein the        anode active material is in ionic contact with a first        electrolyte;    -   (b) a cathode having (i) a cathode active material slurry or        suspension comprising a cathode active material and an optional        conductive additive dispersed in a second liquid or gel        electrolyte, the same as or different than the first liquid or        gel electrolyte, and (ii) a conductive porous structure acting        as a 3D cathode current collector wherein the conductive porous        structure has at least 70% by volume of pores (preferably at        least 80% and more preferably at least 90%) and wherein the        cathode active material slurry is disposed in pores of the        cathode conductive porous structure, wherein the cathode active        material is selected from sulfur, lithium polysulfide, sodium        polysulfide, sulfur-polymer composite, organo-sulfides,        sulfur-carbon composite, sulfur-graphene composite, or a        combination thereof; and    -   (c) a separator disposed between said anode and said cathode;        wherein the cathode thickness-to-cathode current collector        thickness ratio is from 0.8/1 to 1/0.8, and/or the cathode        active material constitutes an electrode active material loading        greater than 15 mg/cm², and the 3D cathode current collector has        a thickness no less than 200 μm (preferably greater than 300 μm,        more preferably greater than 400 μm, further preferably greater        than 500 μm, and most preferably greater than 600 μm). There is        no theoretical limit on the thickness of the conductive porous        structure. A thicker porous structure (or porous current        collector) implies a greater amount of electrode active        materials. Given the same separator layer and approximately the        same packaging envelop and other non-active components, this        thicker electrode also implies a relatively higher proportion of        active materials and, hence, higher energy density.

In an alkali metal-sulfur battery (e.g. wherein the anode activematerial is a Li foil or Na foil), the anode current collector maycontain a porous foamed structure. In an alkali metal-sulfur battery,the first electrolyte can be a gel electrolyte or solid-stateelectrolyte.

In certain embodiments, the cathode active material is supported by afunctional material or nano-structured material selected from the groupconsisting of: (a) A nano-structured or porous disordered carbonmaterial selected from particles of a soft carbon, hard carbon,polymeric carbon or carbonized resin, meso-phase carbon, coke,carbonized pitch, carbon black, activated carbon, nano-cellular carbonfoam or partially graphitized carbon; (b) A nano graphene plateletselected from a single-layer graphene sheet or multi-layer grapheneplatelet; (c) A carbon nanotube selected from a single-walled carbonnanotube or multi-walled carbon nanotube; (d) A carbon nano-fiber,nano-wire, metal oxide nano-wire or fiber, conductive polymernano-fiber, or a combination thereof; (e) A carbonyl-containing organicor polymeric molecule; (f) A functional material containing a carbonyl,carboxylic, or amine group to reversibly capture sulfur; andcombinations thereof.

In certain embodiments, the anode active material contains an alkali ionsource selected from an alkali metal, an alkali metal alloy, a mixtureof alkali metal or alkali metal alloy with an alkali intercalationcompound, an alkali element-containing compound, or a combinationthereof.

In some embodiments (e.g. Li ion-sulfur or sodium ion-sulfur cell), theanode active material contains an alkali intercalation compound selectedfrom petroleum coke, carbon black, amorphous carbon, activated carbon,hard carbon, soft carbon, templated carbon, hollow carbon nanowires,hollow carbon sphere, natural graphite, artificial graphite, lithium orsodium titanate, NaTi₂(PO₄)₃, Na₂Ti₃O₇, Na₂C₈H₄O₄, Na₂TP, Na_(x)TiO₂(x=0.2 to 1.0), Na₂C₈H₄O₄, carboxylate based materials, C₈H₄Na₂O₄,C₈H₆O₄, C₈H₅NaO₄, C₈Na₂F₄O₄,C_(1o)H₂Na₄O₈, C₁₄H₄O₆, C₁₄H₄Na₄O₈, or acombination thereof.

In some embodiments, the anode active material contains an alkaliintercalation compound or alkali-containing compound selected from thefollowing groups of materials: (A) Lithium- or sodium-doped silicon(Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi),zinc (Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni),manganese (Mn), cadmium (Cd), and mixtures thereof; (B) Lithium- orsodium-containing alloys or intermetallic compounds of Si, Ge, Sn, Pb,Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (C) Lithium- orsodium -containing oxides, carbides, nitrides, sulfides, phosphides,selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof; (D) Lithiumor sodium salts; and (E) Graphene sheets pre-loaded with lithium orsodium.

The graphene sheets pre-loaded with lithium or sodium may be selectedfrom pre-sodiated or pre-lithiated versions of pristine graphene,graphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, boron-doped graphene, nitrogen-doped graphene,chemically functionalized graphene, a physically or chemically activatedor etched version thereof, or a combination thereof.

The first or second electrolyte may be selected from aqueouselectrolyte, an organic electrolyte, ionic liquid electrolyte, mixtureof an organic electrolyte and an ionic electrolyte, or a mixture thereofwith a polymer. In an embodiment, the aqueous electrolyte contains asodium salt or a potassium salt dissolved in water or a mixture of waterand alcohol. The sodium salt or lithium salt may be selected fromNa₂SO₄, Li₂SO₄, NaOH, LiOH, NaCl, LiCl, NaF, LiF, NaBr, LiBr, NaI, LiI,or a mixture thereof.

The alkali metal-sulfur battery may contain an organic electrolytehaving a liquid organic solvent selected from the group consisting of1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycoldimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME),diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE),sulfone, sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma.-butyrolactone(γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF),methyl formate (MF), toluene, xylene, methyl acetate (MA),fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethylcarbonate (AEC), a hydrofloroether, and combinations thereof.

The electrolyte in the alkali metal-sulfur battery may contain an alkalimetal salt selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂, Lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), Lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF3(CF₂CF₃)₃), lithiumbisperfluoroethysulfonyl-imide (LiBETI), sodium perchlorate (NaClO₄),potassium perchlorate (KClO₄), sodium hexafluorophosphate (NaPF₆),potassium hexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄),potassium borofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

The alkali metal-sulfur battery may contain an ionic liquid electrolytecontaining an ionic liquid solvent selected from a room temperatureionic liquid having a cation selected from tetra-alkylammonium, di-,tri-, or tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, trialkylsulfonium, or acombination thereof. The ionic liquid solvent may be selected from aroom temperature ionic liquid having an anion selected from BF₄ ⁻,B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻,n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻,N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂, AlCl₄ ⁻, F(HF)_(2.3)⁻, or a combination thereof.

The conductive porous structure can be a foam structure comprising aninterconnected 2D or 3D network of electron-conducting paths. This canbe, for instance, end-connected 2D mats, webs, chicken wire-like metalscreens, etc., as illustrated in FIG. 2. This can also be metal foam,conductive polymer foam, graphite foam, carbon foam, or graphene foam,etc., wherein pore walls contain conductive materials.

In a preferred embodiment, as illustrated in FIG. 1(C) or 1(D), the 3Dporous anode current collector extends all the way to an edge of theporous separator layer and in physical contact therewith. The 3D porousconductive cathode current collector may also extend all the way to theopposite edge of the porous separator and in physical contact therewith.In other words, the pore walls of the anode current collector cover theentire anode layer, and/or the pore walls of the cathode currentcollector cover the entire cathode layer. In these configurations, theratio of current collector thickness/active material layer thickness isapproximately 1/1 and the electrode thickness is essentially identicalto the current collector thickness (the cathode thickness-to-cathodecurrent collector thickness ratio is approximately 1 and the anodethickness-to-anode current collector thickness ratio is approximately1). In these situations, conductive pore walls are in the immediatevicinity of every anode active material particle or every cathode activematerial particle.

In certain embodiments, the ratio of current collector thickness/activematerial layer thickness can be from approximately 0.8/1.0 to 1.0/0.8.Expressed in an alternative manner, the cathode thickness-to-cathodecurrent collector thickness ratio is from 0.8/1 to 1/0.8 or the anodethickness-to-anode current collector thickness ratio is from 0.8/1 to1/0.8. It may be noted that in a conventional lithium-ion or sodium-ionbattery (as schematically illustrated in FIGS. 1(A) and 1(B)), the anode(or cathode) current collector is typically a Cu foil (or Al foil) thatis 8-12 μm thick. The anode active material layer coated on the Cu foilsurface is typically 80-100 μm. As such, the ratio of anode currentcollector thickness/anode active material layer thickness is typically8/100-12/80. The ratio of current collector thickness to active materiallayer thickness at the cathode side of a conventional Li-ion or Na-ioncell is also approximately 1/12.5-1/6.7. In contrast, in the inventedbatteries, the ratio is from 0.8/1 to 1/0.8, more desirably 0.9/1 to1/0.9, further more desirably 0.95/1 to 1/0.95, and most desirably andtypically 1/1.

The pore volume (e.g. >70%) of a foamed current collector is acritically important requirement to ensure a large proportion of activematerials accommodated in the current collector. Based on thiscriterion, conventional paper or textiles made of natural and/orsynthetic fibers do not meet this requirement since they do not have asufficient amount of properly sized pores.

The pore sizes in the first and/or second conductive porous structureare preferably in the range from 10 nm to 100 μm, more preferably from100 nm to 50 μm, further preferably from 500 nm to 20 μm, and even morepreferably from 1 μm to 10 μm, and most preferably from 1 μm to 5 μm.These pore size ranges are designed to accommodate anode activematerials (such as carbon particles) and cathode active materials (suchas sulfur/graphene composite particles), having a primary or secondaryparticle size typically from 10 nm to 20 μm in diameter, and mosttypically from 50 nm to 10 μm, further typically from 100 nm to 5 μm,and most typically from 200 nm to 3 μm.

More significantly, however, since all active material particles in apore (e.g. with pore size of 5 μm) are, on average, within a distance of2.5 μm from a pore wall in the 3D foam structure, electrons can bereadily collected from the anode active material particle and Na or Liions do not have to undergo a long-distance solid-state diffusion. Thisis in contrast to the notion that some electrons in the conventionalthick electrode of prior art lithium-ion or sodium-ion battery (e.g.wherein graphite particle layer 100 μm in thickness is coated onto asurface of a solid Cu foil current collector 10 μm thick) must travel atleast 50 μm to get collected by a current collector (meaning a largerinternal resistance and reduced ability to deliver a higher power).

In general, the first liquid electrolyte and the second liquidelectrolyte are identical in a battery, but they can be different incomposition. The liquid electrolytes can be an aqueous liquid, organicliquid, ionic liquid (ionic salt having a melting temperature lower than100 ° C., preferably lower than room temperature, 25° C.), or a mixtureof an ionic liquid and an organic liquid at a ratio from 1/100 to 100/1.The organic liquid is desirable, but the ionic liquid is preferred. Agel electrolyte can also be used provided the electrolyte has someflowability to enable injection. Some small amount 0.1% to 10% can beincorporated into the liquid electrolyte.

In certain embodiments, the 3D porous anode current collector or 3Dporous cathode current collector contains a conductive foam structurehaving a thickness no less than 200 μm, having at least 85% by volume ofpores, and/or the anode active material has a mass loading no less than20 mg/cm², occupies at least 25% by weight or by volume of the entirebattery cell, and/or the cathode active material has a mass loading noless than 20 mg/cm².

In some preferred embodiments, the 3D porous anode current collector or3D porous cathode current collector contains a conductive foam structurehaving a thickness no less than 300 μm, at least 90% by volume of pores,and/or the anode active material has a mass loading no less than 25mg/cm², occupies at least 30% by weight or by volume of the entirebattery cell, and/or the cathode active material has a mass loading noless than 25 mg/cm².

In some further preferred embodiments, the 3D porous anode currentcollector or 3D porous cathode current collector contains a conductivefoam structure having a thickness no less than 400 μm, having at least95% by volume of pores, and/or the anode active material has a massloading no less than 30 mg/cm², occupies at least 35% by weight or byvolume of the entire battery cell, and/or the cathode active materialhas a mass loading no less than 30 mg/cm².

The 3D porous anode current collector or 3D porous cathode currentcollector may contain a conductive foam structure selected from metalfoam, metal web or screen, perforated metal sheet-based 3-D structure,metal fiber mat, metal nanowire mat, conductive polymer nano-fiber mat,conductive polymer foam, conductive polymer-coated fiber foam, carbonfoam, graphite foam, carbon aerogel, carbon xerox gel, graphene foam,graphene oxide foam, reduced graphene oxide foam, carbon fiber foam,graphite fiber foam, exfoliated graphite foam, or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic of a prior art Li—S or Na—S battery cell composed ofan anode current collector, an anode electrode (e.g. Li foil or thin Sncoating layer), a porous separator, a cathode electrode, and a cathodecurrent collector;

FIG. 1(B) Schematic of a prior art sodium-ion battery, wherein theelectrode layer is composed of discrete particles of an active material(e.g. hard carbon particles in the anode layer or polysulfide particlesin the cathode layer).

FIG. 1(C) Schematic of a presently invented lithium-sulfur orsodium-sulfur battery cell, comprising an anode current collector in theform of a highly porous foam, a porous separator, and a cathode currentcollector in the form of a highly porous foam. Suspensions are beinginjected or impregnated into pores of the two current collectors. Halfof the pores have been filled, for illustration purpose.

FIG. 1(D) Schematic of a presently invented Na ion-sulfur or Liion-sulfur battery cell, comprising an anode current collector in theform of a highly conductive porous foam, a porous separator, and acathode current collector in the form of a highly porous foam. The poresof the two foamed current collectors have been impregnated with theirrespective suspensions.

FIG. 1(E) Schematic of a presently invented Na metal-sulfur or Limetal-sulfur battery cell, comprising an anode current collectorcontaining a layer of Na or Li metal or alloy deposited thereon, aporous separator, and a cathode current collector in the form of ahighly porous foam. The pores of this foamed current collector have beenimpregnated with a cathode-electrolyte suspension.

FIG. 1(F) Schematic of a presently invented alkali metal-sulfur batterycell, comprising (i) a stack of alternatingly configured conductiveporous layers and wet anode layers, (ii) a porous separator, and (iii) astack of alternatingly configured conductive porous layers and wetcathode layers prior to consolidation (e.g. via compression) of multipleporous layers (as a backbone of a current collector) and wet electrodelayers together to achieve infiltration of porous layers by the anodeactive material and liquid electrolyte (and optional conductiveadditive) to form the anode electrode and prior to infiltration ofporous layers by the cathode active material and liquid electrolyte (andoptional conductive additive) to form the cathode electrode.

FIG. 1(G) Schematic of a presently invented alkali metal-sulfur batterycell, as illustrated in FIG. 1(F), but after consolidation (e.g. viacompression) of multiple porous layers (as a backbone of a currentcollector) and wet electrode layers together to achieve infiltration ofporous layers by the anode active material and liquid electrolyte (andoptional conductive additive) to form the anode electrode and afterinfiltration of porous layers by the cathode active material and liquidelectrolyte (and optional conductive additive) to form the cathodeelectrode

FIG. 1(H) Schematic of a presently invented alkali metal-sulfur batterycell, comprising an anode current collector containing a layer of alkalimetal (Na or Li or alkali metal alloy deposited thereon, a porousseparator, and a stack of alternatingly placed conductive porous layersand cathode layers before (top portion) and after (lower portion) ofconsolidation and infiltration of cathode active material and liquidelectrolyte into pores of the porous layers.

FIG. 2 Schematic of a foamed or porous current collector, as an example,composed of 5 sheets of highly porous 2D webs (e.g. chicken wire-shapedthin 2D structures) that are end-connected to form a tab (electricalterminal).

FIG. 3(A) Examples of conductive porous layers: metal grid/mesh andcarbon nano-fiber mat.

FIG. 3(B) Examples of conductive porous layers: graphene foam and carbonfoam.

FIG. 3(C) Examples of conductive porous layers: graphite foam and Nifoam.

FIG. 3(D) Examples of conductive porous layers: Cu foam and stainlesssteel foam.

FIG. 4(A) Schematic of a commonly used process for producing exfoliatedgraphite, expanded graphite flakes (thickness >100 nm), and graphenesheets (thickness <100 nm, more typically <10 nm, and can be as thin as0.34 nm).

FIG. 4(B) Schematic drawing to illustrate the processes for producingexfoliated graphite, expanded graphite flakes, and graphene sheets.

FIG. 5 Ragone plots (gravimetric and volumetric power density vs. energydensity) of Na ion-sulfur battery cells containing hard carbon particlesas the anode active material and carbon/sodium polysulfide particles asthe cathode active materials. Two of the 4 data curves are for the cellsprepared according to an embodiment of instant invention and the othertwo by the conventional slurry coating of electrodes (roll-coating).

FIG. 6 Ragone plots (both gravimetric and volumetric power density vs.gravimetric and volumetric energy density) of two Na—S cells, bothcontaining graphene-embraced Na nano particles as the anode activematerial and sulfur coated on graphene sheets as the cathode activematerial. The data are for both sodium ion cells prepared by thepresently invented method and those by the conventional slurry coatingof electrodes.

FIG. 7 Ragone plots of Li—S batteries containing a lithium foil as theanode active material, graphene sheet-supported sulfur as the cathodeactive material, and lithium salt (LiPF₆)-PC/DEC as organic liquidelectrolyte. The data are for both lithium metal-sulfur cells preparedby the presently invented method and those by the conventional slurrycoating of electrodes.

FIG. 8 Ragone plot of a series of Li ion-S cells (graphene-wrapped Sinano particles) prepared by the conventional slurry coating process andthe Ragone plot of corresponding cells prepared by the presentlyinvented process.

FIG. 9 The cell-level gravimetric (Wh/kg) and volumetric energydensities (Wh/L) of Li ion-S cell (Pre-lithiated graphiteanode+graphene-supported S cathode) plotted over the achievable cathodethickness range of the S/RGO cathode prepared via the conventionalmethod without delamination and cracking and those by the presentlyinvented method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is directed at an alkali metal-sulfur battery (Li—S orroom temperature Na—S) exhibiting an exceptionally high volumetricenergy density that has never been previously achieved for the same typeof battery. This does not include the so-called high-temperature Na—Scell that must operate at a temperature higher than the melting point ofthe electrolyte (typically >350° C.) and higher than the melting pointof sulfur. This alkali metal battery can be a primary battery, but ispreferably a secondary battery selected from an alkali metal-ion battery(e.g. using a Li or Na intercalation compound, such as hard carbonparticles) or an alkali metal secondary battery (e.g. using Na or Limetal foil as an anode active material). The battery is based on anaqueous electrolyte, an organic electrolyte, a gel electrolyte, an ionicliquid electrolyte, or a mixture of organic and ionic liquid. The shapeof an alkali metal battery can be cylindrical, square, button-like, etc.The present invention is not limited to any battery shape orconfiguration.

As illustrated in FIG. 1(A) and 1(B), a conventional lithium-ion,sodium-ion, Li—S, or Na—S battery cell is typically composed of an anodecurrent collector (e.g. Cu foil), an anode electrode (anode activematerial layer), a porous separator and/or an electrolyte component, acathode electrode (cathode active material layer), and a cathode currentcollector (e.g. Al foil). In a more commonly used cell configuration(FIG. 1(B)), the anode layer is composed of particles of an anode activematerial (e.g. hard carbon particles), a conductive additive (e.g.expanded graphite flakes), and a resin binder (e.g. SBR or PVDF). Thecathode layer is composed of particles of a cathode active material(e.g. NaFePO₄ particles in a Na-ion cell or S-carbon composite particlesin a Li—S cell), a conductive additive (e.g. carbon black particles),and a resin binder (e.g. PVDF). Both the anode and the cathode layersare typically 60-100 μm thick (typically significantly thinner than 200μm) to give rise to a presumably sufficient amount of current per unitelectrode area. Using an active material layer thickness of 100 μm andthe solid (Cu or Al foil) current collector layer thickness of 10 μm asexamples, the resulting battery configuration has a current collectorthickness-to-active material layer thickness ratio of 10/100 or 1/10 forconventional battery cells.

This thickness range of 60-100 μm is considered an industry-acceptedconstraint under which a battery designer normally works under, based onthe current slurry coating process (roll coating of activematerial-binder-additive mixture slurry). This thickness constraint isdue to several reasons: (a) the existing battery electrode coatingmachines are not equipped to coat excessively thin or excessively thickelectrode layers; (b) a thinner layer is preferred based on theconsideration of reduced lithium ion diffusion path lengths; but, toothin a layer (e.g. <60 μm) does not contain a sufficient amount of anactive alkali metal ion storage material (hence, insufficient currentoutput); (c) thicker electrodes are prone to delaminate or crack upondrying or handling after roll-coating of slurry; and (d) thicker coatingrequires an excessively long heating zone (it is not unusual to have aheating zone longer than 100 meters, making the manufacturing equipmentvery expensive). This constraint has made it impossible to freelyincrease the amount of active materials (those responsible for storingNa or Li ions) without increasing the amounts of all non-activematerials (e.g. current collectors and separator) in order to obtain aminimum overhead weight and a maximum sodium storage capability and,hence, a maximized energy density (Wk/kg or Wh/L of cell).

In a less commonly used cell configuration, as illustrated in FIG. 1(A),either the anode active material (e.g. NaTi₂(PO₄)₃ or Na film) or thecathode active material (e.g. lithium transition metal oxide in a Li-ioncell or sulfur/carbon mixture in a Li—S cell) is deposited in a thinfilm form directly onto a current collector, such as a sheet of copperfoil or Al foil using sputtering. However, such a thin film structurewith an extremely small thickness-direction dimension (typically muchsmaller than 500 nm, often necessarily thinner than 100 nm) implies thatonly a small amount of active material can be incorporated in anelectrode (given the same electrode or current collector surface area),providing a low total Na or Li storage capacity per unit electrodesurface area. Such a thin film must have a thickness less than 100 nm tobe more-resistant to cycling-induced cracking (for the anode) or tofacilitate a full utilization of the cathode active material. Such aconstraint further diminishes the total Na or Li storage capacity andthe sodium or lithium storage capacity per unit electrode surface area.Such a thin-film battery has very limited scope of application.

On the anode side, a sputtered NaTi₂(PO₄)₃ layer thicker than 100 nm hasbeen found to exhibit poor cracking resistance during batterycharge/discharge cycles. It takes but a few cycles to get fragmented. Onthe cathode side, a layer of sulfur thicker than 100 nm does not allowlithium or sodium ions to fully penetrate and reach full body of thecathode layer, resulting in a poor cathode active material utilizationrate. A desirable electrode thickness is at least 100 μm (not 100 nm),with individual active material particle having a dimension desirablyless than 100 nm. Thus, these thin-film electrodes (with a thickness<100 nm) directly deposited on a current collector fall short of therequired thickness by three (3) orders of magnitude. As a furtherproblem, all of the cathode active materials are not very conductive toboth electrons and sodium/lithium ions. A large layer thickness impliesan excessively high internal resistance and a poor active materialutilization rate.

In other words, there are several conflicting factors that must beconsidered concurrently when it comes to the design and selection of acathode or anode active material in terms of material type, size,electrode layer thickness, and active material mass loading. Thus far,there has been no effective solution offered by any prior art teachingto these often conflicting problems. We have solved these challengingissues, which have troubled battery designers and electrochemists alikefor more than 30 years, by developing a new process of producing alkalimetal-sulfur batteries as herein disclosed.

The prior art sodium or lithium battery cell is typically made by aprocess that includes the following steps: (a) The first step is mixingparticles of the anode active material (e.g. hard carbon particles), aconductive filler (e.g. expanded graphite flakes), a resin binder (e.g.PVDF) in a solvent (e.g. NMP) to form an anode slurry. On a separatebasis, particles of the cathode active material (e.g. sodium metalphosphate particles for the Na-ion cell and LFP particles for the Li-ioncell), a conductive filler (e.g. acetylene black), a resin binder (e.g.PVDF) are mixed and dispersed in a solvent (e.g. NMP) to form a cathodeslurry. (b) The second step includes coating the anode slurry onto oneor both primary surfaces of an anode current collector (e.g. Cu foil),drying the coated layer by vaporizing the solvent (e.g. NMP) to form adried anode electrode coated on Cu foil. Similarly, the cathode slurryis coated and dried to form a dried cathode electrode coated on Al foil.Slurry coating is normally done in a roll-to-roll manner in a realmanufacturing situation; (c) The third step includes laminating ananode/Cu foil sheet, a porous separator layer, and a cathode/Al foilsheet together to form a 3-layer or 5-layer assembly, which is cut andslit into desired sizes and stacked to form a rectangular structure (asan example of shape) or rolled into a cylindrical cell structure. (d)The rectangular or cylindrical laminated structure is then encased in analuminum-plastic laminated envelope or steel casing. (e) A liquidelectrolyte is then injected into the laminated structure to make asodium-ion or lithium battery cell.

There are several serious problems associated with the process and theresulting sodium-ion cells and lithium-ion battery cells (or Li—S andNa—S cells):

-   -   1) It is very difficult to produce an electrode layer (anode        layer or cathode layer) that is thicker than 100 μm, let alone        200 μm. There are several reasons why this is the case. An        electrode of 100 μm thickness typically requires a heating zone        of 30-100 meters long in a slurry coating facility, which is too        time consuming, too energy intensive, and not cost-effective.        For some electrode active materials, such as metal oxide        particles or sulfur, it has not been possible to produce an        electrode of good structural integrity that is thicker than 100        μm in a real manufacturing environment on a continuous basis.        The resulting electrodes are very fragile and brittle. Thicker        electrodes have a high tendency to delaminate and crack.    -   2) With a conventional process, as depicted in FIG. 1(A), the        actual mass loadings of the electrodes and the apparent        densities for the active materials are too low to achieve a high        energy density. In most cases, the anode active material mass        loading of the electrodes (areal density) is significantly lower        than 15 mg/cm² and the apparent volume density or tap density of        the active material is typically less than 1.2 g/cm³ even for        relatively large particles of graphite. The cathode active        material mass loading of the electrodes (areal density) is        significantly lower than 10 mg/cm² for the sulfur cathode. In        addition, there are so many other non-active materials (e.g.        conductive additive and resin binder) that add additional        weights and volumes to the electrode without contributing to the        cell capacity. These low areal densities and low volume        densities result in a relatively low gravimetric energy density        and low volumetric energy density.    -   3) The conventional process requires dispersing electrode active        materials (anode active material and cathode active material) in        a liquid solvent (e.g. NMP) to make a slurry and, upon coating        on a current collector surface, the liquid solvent has to be        removed to dry the electrode layer. Once the anode and cathode        layers, along with a separator layer, are laminated together and        packaged in a housing to make a supercapacitor cell, one then        injects a liquid electrolyte (using a salt dissolved in a        solvent different than NMP) into the cell. In actuality, one        makes the two electrodes wet, then makes the electrodes dry, and        finally makes them wet again. Such a wet-dry-wet process is not        a good process at all. Furthermore, the most commonly used        solvent (NMP) is a notoriously undesirable solvent (known to        cause birth defect, for instance).    -   4) Current Li—S and Na—S batteries still suffer from a        relatively low gravimetric energy density and low volumetric        energy density. Hence, neither the Li—S nor room temperature        Na—S battery has made it to the market place.

In literature, the energy density data reported based on either theactive material weight alone or the electrode weight cannot directlytranslate into the energy densities of a practical battery cell ordevice. The “overhead weight” or weights of other device components(binder, conductive additive, current collectors, separator,electrolyte, and packaging) must also be taken into account. Theconvention production process results in the weight proportion of theanode active material (e.g. carbon particles) in a sodium-ion batterybeing typically from 15% to 20%, and that of the cathode active material(e.g. sodium transition metal oxide) from 20% to 30%.

The present invention provides a process for producing a Li—S or Na—Sbattery cell having a high electrode thickness (thickness of theelectrode that contains electrode active materials, not including thethickness of any active material-free current collector layer, ifexisting), high active material mass loading, low overhead weight andvolume, high volumetric capacitance, and high volumetric energy density.In one embodiment, as illustrated in FIG. 1(C) and 1(D), the inventedprocess comprises:

-   -   (A) Assembling a porous cell framework composed of a first        conductive porous or foam structure 236 as an anode current        collector, a second conductive porous or foam structure as a        cathode current collector 238, and a porous separator 240        disposed between the first and second conductive porous        structure;        -   a. The first and/or second conductive porous structure has a            thickness no less than 100 μm (preferably greater than 200            μm, more preferably greater than 300 μm, further preferably            greater than 400 μm, and most preferably greater than 500            μm) and at least 70% by volume of pores (preferably at least            80% porosity, more preferably at least 90%, and most            preferably at least 95%);        -   b. These conductive porous structures have essentially a            porosity level of 70%-99% and remaining 1%-30% being pore            walls (e.g. metal or graphite skeleton). These pores are            used to accommodate a mixture of active materials (e.g.            carbon particles in the anode+an optional conductive            additive) and liquid electrolyte.    -   (B) Preparing a first suspension (or slurry) of an anode active        material and an optional conductive additive dispersed in a        first liquid electrolyte and a second suspension (slurry) of a        cathode active material and an optional conductive additive        dispersed in a second liquid electrolyte; and    -   (C) Injecting or impregnating the first suspension into pores of        the first conductive porous structure to form an anode and        injecting or impregnating the second suspension into pores of        the second conductive foam structure to form a cathode to an        extent that the anode active material constitutes an electrode        active material loading no less than 20 mg/cm² (preferably no        less than 25 mg/cm² and more preferably no less than 30 mg/cm²)        in the anode, or the cathode active material constitutes an        electrode active material mass loading no less than 10 mg/cm²        (preferably greater than 15 mg/cm² and more preferably greater        than 20 mg/cm²) for a sulfur-based cathode active material),        wherein the anode, the separator, and the cathode are assembled        in a protective housing.        -   a. Preferably, substantially all of the pores are filled            with the electrode (anode or cathode) active material,            optional conductive additive, and liquid electrolyte (no            binder resin needed).        -   b. Since there are great amounts of pores (70-99%) relative            to the pore walls (1-30%), very little space is wasted            (“being wasted” means not being occupied by the electrode            active material and electrolyte), resulting in high amounts            of electrode active material-electrolyte zones (high active            material loading mass).        -   c. Shown in FIG. 1(C) is a situation, wherein the conductive            porous structure for the anode (3D anode current collector            236) has been partially filled with the first suspension            (anode active material and optional conductive additive            dispersed in the liquid electrolyte). The top portion 240 of            the anode current collector foam 236 remains empty, but the            lower portion 244 has been filled with the anode suspension.            Similarly, the top portion 242 of the cathode current            collector foam 238 remains empty and the lower portion 246            has been filled with the cathode suspension (cathode active            material dispersed in the liquid electrolyte). The four            arrows represent the suspension injection directions.

Shown in FIG. 1(D) is a situation, wherein both the anode currentcollector foam and the cathode current collector foam have been filledwith their respective suspensions. As an example, a foam pore 250, in anenlarged view, is filled with the anode suspension containing hardcarbon particles 252 (an anode active material) and liquid electrolyte254. Similarly, a foam pore 260, in an enlarged view, is filled with thecathode suspension containing carbon-coated sulfur or polysulfideparticles 262 (a cathode active material) and liquid electrolyte 264.

An alternative configuration, as schematically illustrated in FIG. 1(E),is a presently invented sodium metal or lithium metal battery cell,comprising an anode current collector 280 containing a layer of Na or Limetal 282 or Na/Li metal alloy deposited thereon, a porous separator,and a cathode current collector in the form of a highly porous foam. Thepores 270 of this foamed current collector have been impregnated with asuspension of cathode active material 272 and liquid electrolyte 274.

In such configurations (FIG. 1(C)-(E)), the electrons only have totravel a short distance (half of the pore size, on average; e.g. a fewmicrometers) before they are collected by the current collector (porewalls) since pore walls are present everywhere throughout the entirecurrent collector (also the entire anode layer). Additionally, in eachsuspension, all electrode active material particles are pre-dispersed ina liquid electrolyte (no electrolyte wettability issue), eliminating theexistence of dry pockets commonly present in an electrode prepared bythe conventional process of wet coating, drying, packing, andelectrolyte injection. Thus, the presently invented process leads to atotally unexpected advantage over the conventional battery cellproduction process.

The present invention also provides another version of the process forproducing a lithium-sulfur or sodium-sulfur battery cell having a highelectrode thickness, high active material mass loading, low overheadweight and volume, high volumetric capacitance, and high volumetricenergy density. In one embodiment, as illustrated in FIGS. 1(F) and1(G), the invented process comprises:

-   -   (A) Preparing a plurality of electrically conductive porous        layers (e.g. 340 a, 340 b at the anode side and 342 a, 342 b at        the cathode side), a plurality of wet anode layers (e.g. 336 a,        336 b) of an anode active material and an optional conductive        additive mixed with a first liquid electrolyte, and a plurality        of wet cathode layers (e.g. 338 a, 338 b) of a cathode active        material and an optional conductive additive mixed with a second        liquid electrolyte, wherein the conductive porous layers contain        interconnected conductive pathways and at least 70% by volume of        pores (preferably >80%);    -   (B) Stacking and consolidating a desired number of porous layers        and a desired number of wet anode layers in an alternating        manner (e.g. 344, shown prior to consolidation) to form an anode        electrode (e.g. 364 in FIG. 1(G), after consolidation) having a        thickness no less than 100 μm (preferably >200 μm, further        preferably >300 μm, more preferably >400 μm; further more        preferably >500 μm, 600 μm, or even >1,000 μm; no theoretical        limitation on this anode thickness). Consolidation may involve        the application of a compressive stress to force the wet anode        layer ingredients to infiltrate into the pores of the multiple        conductive porous layers. These multiple conductive porous        layers are also compressed together to form an anode current        collector that essentially extends over the thickness of the        entire anode electrode 364;    -   (C) Placing a porous separator layer (341 in FIG. 1(F) or 1(G))        in contact with the anode electrode;    -   (D) Stacking and consolidating a desired number of conductive        porous layers and a desired number of wet cathode layers in an        alternating manner (e.g. 346, shown prior to consolidation) to        form a cathode electrode (e.g. 362 in FIG. 1(G), after        consolidation) in contact with the porous separator 341, wherein        the cathode electrode has a thickness no less than 100 μm;        wherein step (D) is conducted before or after step (B); The        cathode thickness is preferably >200 μm, further preferably >300        μm, more preferably >400 μm; further more preferably >500 μm,        600 μm, or even >1,000 μm; no theoretical limitation on this        cathode thickness. Consolidation may involve the application of        a compressive stress to force the wet cathode layer ingredients        to infiltrate into the pores of the multiple conductive porous        layers. These multiple conductive porous layers are also        compressed together to form a cathode current collector that        essentially extends over the entire thickness of the cathode        electrode 362; and    -   (E) Assembling and sealing the anode electrode, porous        separator, and cathode electrode in a housing to produce the        lithium battery;        In this battery, the anode active material has a material mass        loading no less than 20 mg/cm² in the anode electrode and/or the        cathode active material has a material mass loading no less than        15 mg/cm² in the cathode electrode.

It may be noted that, if we pick a small zone 350 in the anode 364, wewill find anode active material particles 352 (plus optional conductiveadditive) dispersed in a liquid electrolyte. If we pick a small zone 360in the cathode, we will find cathode material particles (plus optionalconductive additive) dispersed in a liquid electrolyte.

An alternative configuration, schematically illustrated in FIG. 1(H), isa presently invented lithium metal-sulfur or sodium metal-sulfur batterycell, comprising an anode current collector 280 containing a layer oflithium or sodium metal 382 or lithium metal alloy (or sodium metalalloy) deposited thereon, a porous separator 384, and a stack 388 ofalternatingly arranged conductive porous layers (e.g. 374 a, 374 b),intended for use as part of a cathode current collector, and wet cathodelayers (e.g. 372 a, 372 b) prior to being consolidated (compressed).Upon consolidation, the cathode active material and the liquidelectrolyte (along with any optional conductive additive) are forced topermeate into pores of the conductive porous layers, which are alsocompressed to form a cathode layer 390. The pores 370 of this foamedcurrent collector have been impregnated with a mixture of cathode activematerial particles and liquid electrolyte.

In a preferred embodiment, the anode active material is a pre-sodiatedor pre-lithiated version of graphene sheets selected from pristinegraphene, graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene, ora combination thereof. The starting graphitic material for producing anyone of the above graphene materials may be selected from naturalgraphite, artificial graphite, meso-phase carbon, meso-phase pitch,meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber,carbon nano-fiber, carbon nano-tube, or a combination thereof. Graphenematerials are also a good conductive additive for both the anode andcathode active materials of an alkali metal battery.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of hexagonal carbon atoms,which are single-atom thick, provided the inter-planar van der Waalsforces can be overcome. An isolated, individual graphene plane of carbonatoms is commonly referred to as single-layer graphene. A stack ofmultiple graphene planes bonded through van der Waals forces in thethickness direction with an inter-graphene plane spacing ofapproximately 0.3354 nm is commonly referred to as a multi-layergraphene. A multi-layer graphene platelet has up to 300 layers ofgraphene planes (<100 nm in thickness), but more typically up to 30graphene planes (<10 nm in thickness), even more typically up to 20graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nano graphene platelets” (NGPs).Graphene sheets/platelets (collectively, NGPs) are a new class of carbonnano material (a 2-D nano carbon) that is distinct from the 0-Dfullerene, the 1-D CNT or CNF, and the 3-D graphite. For the purpose ofdefining the claims and as is commonly understood in the art, a graphenematerial (isolated graphene sheets) is not (and does not include) acarbon nanotube (CNT) or a carbon nano-fiber (CNF).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 4(A) and FIG. 4(B) (schematic drawings). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes in a GIC or GO serves to increase theinter-graphene spacing (d₀₀₂, as determined by X-ray diffraction),thereby significantly reducing the van der Waals forces that otherwisehold graphene planes together along the c-axis direction. The GIC or GOis most often produced by immersing natural graphite powder (100 in FIG.4(B)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent),and another oxidizing agent (e.g. potassium permanganate or sodiumperchlorate). The resulting GIC (102) is actually some type of graphiteoxide (GO) particles if an oxidizing agent is present during theintercalation procedure. This GIC or GO is then repeatedly washed andrinsed in water to remove excess acids, resulting in a graphite oxidesuspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. In order toproduce graphene materials, one can follow one of the two processingroutes after this rinsing step, briefly described below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (106) that typically have athickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,112), as disclosed in our U.S. patent application Ser. No. 10/858,814(Jun. 3, 2004). Single-layer graphene can be as thin as 0.34 nm, whilemulti-layer graphene can have a thickness up to 100 nm, but moretypically less than 10 nm (commonly referred to as few-layer graphene).Multiple graphene sheets or platelets may be made into a sheet of NGPpaper using a paper-making process. This sheet of NGP paper is anexample of the porous graphene structure layer utilized in the presentlyinvented process.

Route 2 entails ultrasonicating the graphite oxide suspension (e.g.graphite oxide particles dispersed in water) for the purpose ofseparating/isolating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation bas been increased from 0.3354 nm in natural graphiteto 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form fully separated, isolated, or discrete graphene oxide(GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% by weightof oxygen.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Pristine graphene, in smaller discrete graphene sheets (typically 0.3 μmto 10 μm), may be produced by direct ultrasonication (also known asliquid phase exfoliation or production) or supercritical fluidexfoliation of graphite particles. These processes are well-known in theart.

The graphene oxide (GO) may be obtained by immersing powders orfilaments of a starting graphitic material (e.g. natural graphitepowder) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid,nitric acid, and potassium permanganate) in a reaction vessel at adesired temperature for a period of time (typically from 0.5 to 96hours, depending upon the nature of the starting material and the typeof oxidizing agent used). As previously described above, the resultinggraphite oxide particles may then be subjected to thermal exfoliation orultrasonic wave-induced exfoliation to produce isolated GO sheets. TheseGO sheets can then be converted into various graphene materials bysubstituting —OH groups with other chemical groups (e.g. —Br, NH₂,etc.).

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished.

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In(CF)_(n), carbon atoms are sp3-hybridized and thus the fluorocarbonlayers are corrugated consisting of trans-linked cyclohexane chairs. In(C₂F)_(n) only half of the C atoms are fluorinated and every pair of theadjacent carbon sheets are linked together by covalent C—C bonds.Systematic studies on the fluorination reaction showed that theresulting F/C ratio is largely dependent on the fluorinationtemperature, the partial pressure of the fluorine in the fluorinatinggas, and physical characteristics of the graphite precursor, includingthe degree of graphitization, particle size, and specific surface area.In addition to fluorine (F₂), other fluorinating agents may be used,although most of the available literature involves fluorination with F₂gas, sometimes in presence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultra-sonic treatmentof a graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 4(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 4(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as graphite worms 104. These wormsof graphite flakes which have been greatly expanded can be formedwithout the use of a binder into cohesive or integrated sheets ofexpanded graphite, e.g. webs, papers, strips, tapes, foils, mats or thelike (typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

Acids, such as sulfuric acid, are not the only type of intercalatingagent (intercalant) that penetrate into spaces between graphene planesto obtain GICs. Many other types of intercalating agents, such as alkalimetals (Li, K, Na, Cs, and their alloys or eutectics), can be used tointercalate graphite to stage 1, stage 2, stage 3, etc. Stage n impliesone intercalant layer for every n graphene planes. For instance, astage-1 potassium-intercalated GIC means there is one layer of K forevery graphene plane; or, one can find one layer of K atoms insertedbetween two adjacent graphene planes in a G/K/G/K/G/KG . . . sequence,where G is a graphene plane and K is a potassium atom plane. Astage-2GIC will have a sequence of GG/K/GG/K/GG/K/GG . . . and a stage-3GIC will have a sequence of GGG/K/GGG/K/GGG . . . , etc. These GICs canthen be brought in contact with water or water-alcohol mixture toproduce exfoliated graphite and/or separated/isolated graphene sheets.

Exfoliated graphite worms may be subjected to high-intensity mechanicalshearing/separation treatments using a high-intensity air jet mill,high-intensity ball mill, or ultrasonic device to produce separated nanographene platelets (NGPs) with all the graphene platelets thinner than100 nm, mostly thinner than 10 nm, and, in many cases, beingsingle-layer graphene (also illustrated as 112 in FIG. 4(B)). An NGP iscomposed of a graphene sheet or a plurality of graphene sheets with eachsheet being a two-dimensional, hexagonal structure of carbon atoms. Amass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide may be madeinto a graphene film/paper (114 in FIG. 4(B)) using a film- orpaper-making process. Alternatively, with a low-intensity shearing,graphite worms tend to be separated into the so-called expanded graphiteflakes (108 in FIG. 4(B) having a thickness >100 nm. These flakes can beformed into graphite paper or mat 106 using a paper- or mat-makingprocess, with or without a resin binder. Expanded graphite flakes can beused as a conductive filler in a battery. Separated NGPs (individualsingle-layer or multi-layer graphene sheets) can be used as an anodeactive material or as a supporting conductive material in the cathode ofan alkali metal-sulfur battery.

There is no restriction on the types of anode active materials orcathode active materials that can be used in practicing the instantinvention. In one preferred embodiment, the anode active material isselected from the group consisting of: (a) Sodium- or lithium-dopedsilicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb),bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), cobalt (Co),nickel (Ni), manganese (Mn), cadmium (Cd), and mixtures thereof; (b)Sodium- or lithium-containing alloys or intermetallic compounds of Si,Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c)Sodium- or lithium-containing oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof;(d) Sodium or lithium salts; and (e) Graphene sheets pre-loaded orpre-attached with sodium or lithium (herein referred to as pre-sodiatedor pre-lithiated graphene sheets).

In the rechargeable alkali metal-sulfur battery, the anode may containan alkali ion source selected from an alkali metal, an alkali metalalloy, a mixture of alkali metal or alkali metal alloy with an alkaliintercalation compound, an alkali element-containing compound, or acombination thereof. Particularly desired is an anode active materialthat contains an alkali intercalation compound selected from petroleumcoke, carbon black, amorphous carbon, hard carbon, templated carbon,hollow carbon nanowires, hollow carbon sphere, natural graphite,artificial graphite, lithium or sodium titanate, NaTi₂(PO₄)₃, Na₂Ti₃O₇(Sodium titanate), Na₂C₈H₄O₄ (Disodium Terephthalate), Na₂TP (SodiumTerephthalate), TiO₂, Na_(x)TiO₂ (x=0.2 to 1.0), carboxylate basedmaterials, C₈H₄Na₂O₄, C₈H₆O₄, C₈H₅NaO₄, C₈Na₂F₄O₄,C₁₀H₂Na₄O₈, C₁₄H₄O₆,C₁₄H₄Na₄O₈, or a combination thereof. In an embodiment, the anode maycontain a mixture of 2 or 3 types of anode active materials (e.g. mixedparticles of activated carbon+NaTi₂(PO₄)₃ or a mixture of Li particlesand graphite particles).

The first or second liquid electrolyte in the invented process orbattery may be selected from an aqueous electrolyte, organicelectrolyte, ionic liquid electrolyte, mixture of an organic electrolyteand an ionic electrolyte, or a mixture thereof with a polymer. In someembodiments, the aqueous electrolyte contains a sodium salt or apotassium salt dissolved in water or a mixture of water and alcohol. Insome embodiments, the sodium salt or potassium salt is selected fromNa₂SO₄, K₂SO₄, a mixture thereof, NaOH, LiOH, NaCl, LiCl, NaF, LiF,NaBr, LiBr, NaI, LiI, or a mixture thereof.

The organic solvent may contain a liquid solvent selected from the groupconsisting of 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma-butyrolactone (y-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene, methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), a hydrofloroether (e.g. methylperfluorobutyl ether, MFE, or ethyl perfluorobutyl ether, EFE), andcombinations thereof.

The organic electrolyte may contain an alkali metal salt preferablyselected from sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), bis-trifluoromethyl sulfonylimide potassium(KN(CF₃SO₂)₂), an ionic liquid salt, or a combination thereof.

The electrolyte may contain a lithium salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyl-difluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimide (LiBETI),lithium bis(trifluoromethanesulphonyl)imide, lithiumbis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid lithium salt, or a combination thereof.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, an ionic salt is considered as anionic liquid if its melting point is below 100° C. If the meltingtemperature is equal to or lower than room temperature (25° C.), thesalt is referred to as a room temperature ionic liquid (RTIL). TheIL-based lithium salts are characterized by weak interactions, due tothe combination of a large cation and a charge-delocalized anion. Thisresults in a low tendency to crystallize due to flexibility (anion) andasymmetry (cation).

Some ILs may be used as a co-solvent (not as a salt) to work with thefirst organic solvent of the present invention. A well-known ionicliquid is formed by the combination of a 1-ethyl-3-methyl-imidazolium(EMI) cation and an N,N-bis(trifluoromethane)sulphonamide (TFSI) anion.This combination gives a fluid with an ionic conductivity comparable tomany organic electrolyte solutions, a low decomposition propensity andlow vapor pressure up to ˜300-400° C. This implies a generally lowvolatility and non-flammability and, hence, a much safer electrolytesolvent for batteries.

Ionic liquids are basically composed of organic or inorganic ions thatcome in an unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide andhexafluorophosphate as anions. Useful ionic liquid-based sodium salts(not solvent) may be composed of sodium ions as the cation andbis(trifluoromethanesulphonyl)imide, bis(fluorosulphonyl)imide orhexafluorophosphate as anions. For instance, sodiumtrifluoromethanesulfonimide (NaTFSI) is a particularly useful sodiumsalt.

Based on their compositions, ionic liquids come in different classesthat include three basic types: aprotic, protic and zwitterionic types,each one suitable for a specific application. Common cations of roomtemperature ionic liquids (RTILs) include, but are not limited to,tetraalkylammonium, di, tri, and tetra-alkylimidazolium,alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium,tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILsinclude, but are not limited to, BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻,CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻,SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc. Relatively speaking,the combination of imidazolium- or sulfonium-based cations and complexhalide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, NTf₂ ⁻,N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs with good workingconductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte co-solvent in a rechargeablelithium cell.

The specific capacity and specific energy of a Li—S cell or Na—S cellare dictated by the actual amount of sulfur that can be implemented inthe cathode active layer (relative to other non-active ingredients, suchas the binder resin and conductive filler) and the utilization rate ofthis sulfur amount (i.e. the utilization efficiency of the cathodeactive material or the actual proportion of S that actively participatesin storing and releasing lithium ions). A high-capacity and high-energyLi—S or Na—S cell requires a high amount of S in the cathode activelayer (i.e. relative to the amounts of non-active materials, such as thebinder resin, conductive additive, and other modifying or supportingmaterials) and a high S utilization efficiency). The present inventionprovides such a cathode active layer and a method of producing such acathode active layer (e.g. a pre-sulfurized active cathode layer). As anexample of sulfur pre-loading procedures, this method comprises thefollowing four steps, (a)-(d):

-   -   a) Preparing a layer of porous graphene structure having massive        graphene surfaces with a specific surface area greater than 100        m²/g (these surfaces must be accessible to electrolyte). The        porous graphene structure have a specific surface area        preferably >500 m²/g and more preferably >700 m²/g, and most        preferably >1,000 m²/g.    -   b) Preparing an electrolyte comprising a solvent (non-aqueous        solvent, such as organic solvent and or ionic liquid) and a        sulfur source dissolved or dispersed in the solvent;    -   c) Preparing an anode;    -   d) Bringing the integral layer of porous graphene structure and        the anode in ionic contact with the electrolyte (e.g. by        immersing all these components in a chamber that is external to        the intended Li—S cell, or encasing these three components        inside the Li—S cell) and imposing an electric current between        the anode and the integral layer of porous graphene structure        (serving as a cathode) with a sufficient current density for a        sufficient period of time to electrochemically deposit        nano-scaled sulfur particles or coating on the graphene surfaces        to form a pre-sulfurized graphene layer;    -   e) Pulverizing this pre-sulfurized layer to produce isolated        S-coated graphene sheets. These sheets can be injected or        impregnated into the pores of a cathode current collector foam        (porous conductive structure) to make the cathode.        The layer of porous graphene structure recited in step (a)        contains a graphene material or an exfoliated graphite material,        wherein the graphene material is selected from pristine        graphene, graphene oxide, reduced graphene oxide, graphene        fluoride, graphene chloride, graphene bromide, graphene iodide,        hydrogenated graphene, nitrogenated graphene, boron-doped        graphene, nitrogen-doped graphene, chemically functionalized        graphene, or a combination thereof, and wherein the exfoliated        graphite material is selected from exfoliated graphite worms,        expanded graphite flakes, or recompressed graphite worms or        flakes (must still exhibit a high specific surface area, >>100        m²/g, accessible to electrolyte). It is surprising to discover        that multiple graphene sheets can be packed together to form a        sulfur-based electrode layer of structural integrity without the        need for a binder resin, and such a layer can hold its shape and        functions during repeated charges and discharges of the        resulting Li—S cell.

The S particles or coating have a thickness or diameter smaller than 20nm (preferably <10 nm, more preferably <5 nm, and further preferably <3nm) and wherein the nano-scaled sulfur particles or coating occupy aweight fraction of at least 70% (preferably >80%, more preferably >90%,and most preferably >95%) based on the total weights of the sulfurparticles or coating and the graphene material combined. It isadvantageous to deposit as much S as possible yet still maintainultra-thin thickness or diameter of the S coating or particles(e.g. >80% and <3 nm; >90% and <5 nm; and >95% and <10 nm, etc.).

Once a layer of porous graphene structure is prepared, this layer can beimmersed in an electrolyte (preferably liquid electrolyte), whichcomprises a solvent and a sulfur source dissolved or dispersed in thesolvent. This layer basically serves as a cathode in an externalelectrochemical deposition chamber.

Subsequently, an anode layer is also immersed in the chamber. Anyconductive material can be used as an anode material, but preferablythis layer contains some lithium or sodium. In such an arrangement, thelayer of porous graphene structure and the anode are in ionic contactwith the electrolyte. An electric current is then supplied between theanode and the integral layer of porous graphene structure (serving as acathode) with a sufficient current density for a sufficient period oftime to electrochemically deposit nano-scaled sulfur particles orcoating on the graphene surfaces to form the pre-sulfurized activecathode layer. The required current density depends upon the desiredspeed of deposition and uniformity of the deposited material.

This current density can be readily adjusted to deposit S particles orcoating that have a thickness or diameter smaller than 20 nm (preferably<10 nm, more preferably <5 nm, and further preferably <3 nm). Theresulting nano-scaled sulfur particles or coating occupy a weightfraction of at least 70% (preferably >80%, more preferably >90%, andmost preferably >95%) based on the total weights of the sulfur particlesor coating and the graphene material combined.

In one preferred embodiment, the sulfur source is selected fromM_(x)S_(y), wherein x is an integer from 1 to 3 and y is an integer from1 to 10, and M is a metal element selected from an alkali metal, analkaline metal selected from Mg or Ca, a transition metal, a metal fromgroups 13 to 17 of the periodic table, or a combination thereof. In adesired embodiment, the metal element M is selected from Li, Na, K, Mg,Zn, Cu, Ti, Ni, Co, Fe, or Al. In a particularly desired embodiment,M_(x)S_(y) is selected from Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S₆,Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂ 5 ₆, K₂ 5 ₇, K₂ 5 ₈, K₂ 5 ₉, or K₂S₁₀.

In one embodiment, the anode comprises an anode active material selectedfrom an alkali metal, an alkaline metal, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof.This anode can be the same anode intended for inclusion in a Li—S cell.

The solvent and lithium or sodium salt used in the electrochemicaldeposition chamber may be selected from any solvent or salt in the listgiven above for a lithium-sulfur or sodium-sulfur battery.

After an extensive and in-depth research effort, we have come to realizethat such a pre-sulfurization surprisingly solves several most criticalissues associated with current Li—S or Na—S cells. For instance, thismethod enables the sulfur to be deposited in a thin coating orultra-fine particle form, thus, providing ultra-short lithium iondiffusion paths and, hence, ultra-fast reaction times for fast batterycharges and discharges. This is achieved while maintaining a relativelyhigh proportion of sulfur (the active material responsible for storinglithium) and, thus, high specific lithium storage capacity of theresulting cathode active layer in terms of high specific capacity(mAh/g, based on the total weight of the cathode layer, including themasses of the active material, S, supporting graphene sheets, binderresin, and conductive filler).

It is of significance to note that one might be able to use a prior artprocedure to deposit small S particles, but not a high S proportion, orto achieve a high proportion but only in large particles or thick filmform. But, the prior art procedures have not been able to achieve bothat the same time. This is why it is such an unexpected and highlyadvantageous thing to obtain a high sulfur loading and yet,concurrently, maintaining an ultra-thin/small thickness/diameter ofsulfur. This has not been possible with any prior art sulfur loadingtechniques. For instance, we have been able to deposit nano-scaledsulfur particles or coating that occupy a >90% weight fraction of thecathode layer and yet maintaining a coating thickness or particlediameter <3 nm. This is quite a feat in the art of lithium-sulfurbatteries. As another example, we have achieved a >95% S loading at anaverage S coating thickness of 4.8-7 nm.

Electrochemists or materials scientists in the art of Li—S batterieswould expect that a greater amount of highly conducting graphene sheetsor graphite flakes (hence, a smaller amount of S) in the cathode activelayer should lead to a better utilization of S, particularly under highcharge/discharge rate conditions. Contrary to these expectations, wehave observed that the key to achieving a high S utilization efficiencyis minimizing the S coating or particle size and is independent of theamount of S loaded into the cathode provided the S coating or particlethickness/diameter is small enough (e.g. <10 nm, or even better if <5nm). The problem here is that it has not been possible to maintain athin S coating or small particle size if S is higher than 50% by weight.Here we have further surprisingly observed that the key to enabling ahigh specific capacity at the cathode under high rate conditions is tomaintain a high S loading and still keep the S coating or particle sizeas small as possible, and this is accomplished by using the presentlyinvented pre-sulfurization method.

The electrons coming from or going out through the external load orcircuit must go through the conductive additives (in a conventionalsulfur cathode) or a conductive framework (e.g. exfoliated graphitemeso-porous structure or nano-structure of conductive graphene sheets asherein disclosed) to reach the cathode active material. Since thecathode active material (e.g. sulfur or lithium polysulfide) is a poorelectronic conductor, the active material particle or coating must be asthin as possible to reduce the required electron travel distance.

Furthermore, the cathode in a conventional Li—S cell typically has lessthan 70% by weight of sulfur in a composite cathode composed of sulfurand the conductive additive/support. Even when the sulfur content in theprior art composite cathode reaches or exceeds 70% by weight, thespecific capacity of the composite cathode is typically significantlylower than what is expected based on theoretical predictions. Forinstance, the theoretical specific capacity of sulfur is 1,675 mAh/g. Acomposite cathode composed of 70% sulfur (S) and 30% carbon black (CB),without any binder, should be capable of storing up to 1,675×70%=1,172mAh/g. Unfortunately, the observed specific capacity is typically lessthan 75% or 879 mAh/g (often less than 50% or 586 mAh/g in this example)of what could be achieved. In other words, the active materialutilization rate is typically less than 75% (or even <50%). This hasbeen a major issue in the art of Li—S cells and there has been nosolution to this problem. Most surprisingly, the implementation ofmassive graphene surfaces associated with a porous graphene structure asa conductive supporting material for sulfur or lithium polysulfide hasmade it possible to achieve an active material utilization rate oftypically >>80%, more often greater than 90%, and, in many cases, closeto 95%-99%.

Alternatively, the cathode active material (S or polysulfide) may bedeposited on or bonded by a functional material or nano-structuredmaterial. The functional material or nano-structured material may beselected from the group consisting of (a) a nano-structured or porousdisordered carbon material selected from a soft carbon, hard carbon,polymeric carbon or carbonized resin, meso-phase carbon, coke,carbonized pitch, carbon black, activated carbon, nano-cellular carbonfoam or partially graphitized carbon; (b) a nano graphene plateletselected from a single-layer graphene sheet or multi-layer grapheneplatelet; (c) a carbon nanotube selected from a single-walled carbonnanotube or multi-walled carbon nanotube; (d) a carbon nano-fiber,nano-wire, metal oxide nano-wire or fiber, conductive polymernano-fiber, or a combination thereof; (e) a carbonyl-containing organicor polymeric molecule; (f) a functional material containing a carbonyl,carboxylic, or amine group; and combinations thereof. In a preferredembodiment, the functional material or nano-structured material has aspecific surface area of at least 500 m²/g, preferably at least 1,000m²/g.

Typically, the cathode active materials are not electrically conducting.Hence, in one embodiment, the cathode active material may be mixed witha conductive filler, such as carbon black (CB), acetylene black (AB),graphite particles, expanded graphite particles, activated carbon,meso-porous carbon, meso-carbon micro bead (MCMB), carbon nano-tube(CNT), carbon nano-fiber (CNF), graphene sheet (also referred to as nanographene platelet, NGP), carbon fiber, or a combination thereof. Thesecarbon/graphite/graphene materials, containing sulfur or polysulfide,may be made into fine particles as the cathode active material in theinvented Li—S or Na—S cell.

In a preferred embodiment, the nano-scaled filaments (e.g. CNTs, CNFs,and/or NGPs) are formed into a porous nano-structure that containsmassive surfaces to support either the anode active material (e.g. Na orLi coating) or the cathode active material (e.g. S). The porousnano-structure should have pores having a pore size preferably from 2 nmto 50 nm, preferably 2 nm-10 nm. These pores are properly sized toaccommodate the electrolyte at the cathode side and to retain thecathode active material in the pores during repeated charges/discharges.The same type of nano-structure may be implemented at the anode side tosupport the anode active material.

At the anode side, when an alkali metal is used as the sole anode activematerial in an alkali metal cell, there is concern about the formationof dendrites, which could lead to internal shorting and thermal runaway.Herein, we have used two approaches, separately or in combination, toaddressing this dendrite formation issue: one involving the use of ahigh-concentration electrolyte and the other the use of a nano-structurecomposed of conductive nano-filaments to support the alkali metal at theanode. The nano-filament may be selected from, as examples, a carbonnano fiber (CNF), graphite nano fiber (GNF), carbon nano-tube (CNT),metal nano wire (MNW), conductive nano-fibers obtained byelectro-spinning, conductive electro-spun composite nano-fibers,nano-scaled graphene platelet (NGP), or a combination thereof. Thenano-filaments may be bonded by a binder material selected from apolymer, coal tar pitch, petroleum pitch, meso-phase pitch, coke, or aderivative thereof.

Surprisingly and significantly, the nano-structure provides anenvironment that is conducive to uniform deposition of alkali metal ionsduring the battery re-charge, to the extent that no geometrically sharpstructures or dendrites were found in the anode after a large number ofcycles. Not wishing to be bound by any theory, but the applicantsenvision that the 3-D network of highly conductive nano-filamentsprovide a substantially uniform attraction of alkali metal ions backonto the filament surfaces during re-charging. Furthermore, due to thenanometer sizes of the filaments, there is a large amount of surfacearea per unit volume or per unit weight of the nano-filaments. Thisultra-high specific surface area offers the alkali metal ions anopportunity to uniformly deposit a thin coating on filament surfaces.The high surface area readily accepts a large amount of alkali metalions in the liquid electrolyte, enabling high re-charge rates for analkali metal secondary battery.

EXAMPLES

In the examples discussed below, unless otherwise noted, raw materialssuch as silicon, germanium, bismuth, antimony, zinc, iron, nickel,titanium, cobalt, and tin were obtained from either Alfa Aesar of WardHill, Mass., Aldrich Chemical Company of Milwaukee, Wis. or Alcan MetalPowders of Berkeley, CA. X-ray diffraction patterns were collected usinga diffractometer equipped with a copper target x-ray tube and adiffracted beam monochromator. The presence or absence of characteristicpatterns of peaks was observed for each of the alloy samples studied.For example, a phase was considered to be amorphous when the X-raydiffraction pattern was absent or lacked sharp, well-defined peaks. Inseveral cases, scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) were used to characterize the structure andmorphology of the hybrid material samples.

In what follows, we provide some examples of several different types ofanode active materials, cathode active materials, and porous currentcollector materials (e.g. graphite foam, graphene foam, and metal foam)to illustrate the best mode of practicing the instant invention.

Theses illustrative examples and other portions of instant specificationand drawings, separately or in combinations, are more than adequate toenable a person of ordinary skill in the art to practice the instantinvention. However, these examples should not be construed as limitingthe scope of instant invention.

Example 1 Illustrative Examples of Conductive Porous Layers (FoamedCurrent Collectors)

Various types of metal foams and fine metal webs/screens arecommercially available; e.g. Ni foam, Cu foam, Al foam, Ti foam, Nimesh/web, stainless steel fiber mesh, etc. These conductive foamstructures were used in the present study as an anode or cathodeconductive porous layers (foam current collectors). In addition,metal-coated polymer foams and carbon foams were also used as currentcollectors.

Example 2 Ni Foam and CVD Graphene Foam-Based Current Collectors(Conductive Porous Layers) on Ni Foam Templates

The procedure for producing CVD graphene foam was adapted from thatdisclosed in open literature: Chen, Z. et al. “Three-dimensionalflexible and conductive interconnected graphene networks grown bychemical vapor deposition,” Nature Materials, 10, 424-428 (2011). Nickelfoam, a porous structure with an interconnected 3D scaffold of nickelwas chosen as a template for the growth of graphene foam. Briefly,carbon was introduced into a nickel foam by decomposing CH₄ at 1,000° C.under ambient pressure, and graphene films were then deposited on thesurface of the nickel foam. Due to the difference in the thermalexpansion coefficients between nickel and graphene, ripples and wrinkleswere formed on the graphene films. Four types of foams made in thisexample were used as a current collector in the presently inventedlithium batteries: Ni foam, CVD graphene-coated Ni form, CVD graphenefoam (Ni being etched away), and conductive polymer bonded CVD graphenefoam.

In order to recover (separate) graphene foam from the supporting Nifoam, Ni frame was etched away. In the procedure proposed by Chen, etal., before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly (methyl methacrylate) (PMMA) wasdeposited on the surface of the graphene films as a support to preventthe graphene network from collapsing during nickel etching. After thePMMA layer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer was consideredcritical to preparing a free-standing film of graphene foam. Instead, weused a conducting polymer as a binder resin to hold graphene togetherwhile Ni was etched away. It may be noted that the CVD graphene foamused herein is intended as a foamed current collector to accommodate asuspension of active material dispersed in a liquid electrolyte. Forinstance, hard carbon nano particles were injected along with a liquidelectrolyte in the anode and graphene-supported sulfur nano particlesinjected along with a liquid electrolyte in the cathode.

Example 3 Graphitic Foam-Based Current Collectors From Pitch-BasedCarbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 meso-phase pitchwas utilized. The sample is evacuated to less than 1 ton and then heatedto a temperature approximately 300° C. At this point, the vacuum wasreleased to a nitrogen blanket and then a pressure of up to 1,000 psiwas applied. The temperature of the system was then raised to 800° C.This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon.

Example 4 Some Examples of Electrolytes Used

Preferred non-lithium alkali metal salts include: sodium perchlorate(NaClO₄), potassium perchlorate (KClO₄), sodium hexafluorophosphate(NaPF₆), potassium hexafluorophosphate (KPF₆), sodium borofluoride(NaBF₄), potassium borofluoride (KBF₄), sodium hexafluoroarsenide,potassium hexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃),potassium trifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethylsulfonylimide sodium (NaN(CF₃SO₂)₂), and bis-trifluoromethylsulfonylimide potassium [KN(CF₃SO₂)₂]. For aqueous electrolyte, sodiumsalt or potassium salt is preferably selected from

Na₂SO₄, K₂SO₄, a mixture thereof, NaOH, KOH, NaCl, KCl, NaF, KF, NaBr,KBr, NaI, KI, or a mixture thereof. The salt concentrations used in thepresent study were from 0.3M to 3.0 M (most often 0.5M to 2.0M).

A wide range of lithium salts dissolved in an organic liquid solvent(alone or in a mixture with another organic liquid or an ionic liquid)were used in the present study. We observed that the following lithiumsalts could be dissolved well in selected organic or ionic liquidsolvents: lithium borofluoride (LiBF₄), lithium trifluoro-metasulfonate(LiCF₃SO₃), lithium bis-trifluoromethyl sulfonylimide (LiN(CF₃SO₂)₂ orLITFSI), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), and lithiumbisperfluoroethy-sulfonylimide (LiBETI). A good electrolyte additive forhelping to stabilize Li metal is LiNO₃. Particularly useful ionicliquid-based lithium salts include: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

Preferred organic liquid solvents include: ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), propylene carbonate (PC), acetonitrile (AN), vinylene carbonate(VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),Poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), hydrofloroether (e.g. TPTP),sulfone, and sulfolane.

Preferred ionic liquid solvents may be selected from a room temperatureionic liquid (RTIL) having a cation selected from tetraalkylammonium,di-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, ordialkylpiperidinium. The counter anion is preferably selected from BF₄⁻, B(CN)₄ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ , N(COCF₃)(SO₂CF₃)⁻, orN(SO₂F)₂ ⁻. Particularly useful ionic liquid-based solvents includeN-n-butyl-N-ethylpyrrolidinium bis(trifluoromethane sulfonyl)imide(BEPyTFSI), N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide (PP₁₃TFSI), and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide.

Example 4 Preparation of Graphene Oxide (GO) and Reduced Graphene Oxide(RGO) Nano Sheets From Natural Graphite Powder

Natural graphite, nominally sized at 45 μm, provided by Asbury Carbons(405 Old Main St., Asbury, N.J. 08802, USA) was milled to reduce thesize to approximately 14 μm, which was used as the starting material. GOwas obtained by following the well-known modified Hummers method, whichinvolved two oxidation stages. In a typical procedure, the firstoxidation was achieved in the following conditions: 1100 mg of graphitewas placed in a 1000 mL boiling flask. Then, 20 g of K₂S₂O₈, 20 g ofP₂O₅, and 400 mL of a concentrated aqueous solution of H₂SO₄ (96%) wereadded in the flask. The mixture was heated under reflux for 6 hours andthen let without disturbing for 20 hours at room temperature. Oxidizedgraphite was filtered and rinsed with abundant distilled water untilneutral pH. A wet cake-like material was recovered at the end of thisfirst oxidation.

For the second oxidation process, the previously collected wet cake wasplaced in a boiling flask that contains 69 mL of a concentrated aqueoussolution of H₂SO₄ (96%). The flask was kept in an ice bath as 9 g ofKMnO₄ was slowly added. Care was taken to avoid overheating. Theresulting mixture was stirred at 35° C. for 2 hours (the sample colorturning dark green), followed by the addition of 140 mL of water. After15 min, the reaction was halted by adding 420 mL of water and 15 mL ofan aqueous solution of 30 wt % H₂O₂. The color of the sample at thisstage turned bright yellow. To remove the metallic ions, the mixture wasfiltered and rinsed with a 1:10 HCl aqueous solution. The collectedmaterial was gently centrifuged at 2700 g and rinsed with deionizedwater. The final product was a wet cake that contained 1.4 wt % of GO,as estimated from dry extracts. Subsequently, liquid dispersions of GOplatelets were obtained by lightly sonicating wet-cake materials, whichwere diluted in deionized water.

Surfactant-stabilized RGO (RGO-BS) was obtained by diluting the wet-cakein an aqueous solution of surfactants instead of pure water. Acommercially available mixture of cholate sodium (50 wt. %) anddeoxycholate sodium (50 wt. %) salts provided by Sigma Aldrich was used.The surfactant weight fraction was 0.5 wt. %. This fraction was keptconstant for all samples. Sonication was performed using a BransonSonifier S-250A equipped with a 13 mm step disruptor horn and a 3 mmtapered micro-tip, operating at a 20 kHz frequency. For instance, 10 mLof aqueous solutions containing 0.1 wt. % of GO was sonicated for 10 minand subsequently centrifuged at 2700 g for 30 min to remove anynon-dissolved large particles, aggregates, and impurities. Chemicalreduction of as-obtained GO to yield RGO was conducted by following themethod, which involved placing 10 mL of a 0.1 wt. % GO aqueous solutionin a boiling flask of 50 mL. Then, 10 μL of a 35 wt. % aqueous solutionof N₂H₄ (hydrazine) and 70 mL of a 28 wt. % of an aqueous solution ofNH₄OH (ammonia) were added to the mixture, which was stabilized bysurfactants. The solution was heated to 90° C. and refluxed for 1 h. ThepH value measured after the reaction was approximately 9. The color ofthe sample turned dark black during the reduction reaction.

RGO was used as a conductive additive in either or both of the anode andcathode in certain alkali metal batteries presently invented.Pre-sodiated RGO (e.g. RGO+sodium particles or RGO pre-deposited withsodium coating) was also use as an anode active material in selectedsodium-sulfur cells. Pre-lithiated RGO films were also used as an anodeactive material for the Li—S cells.

For comparison purposes, slurry coating and drying procedures wereconducted to produce conventional electrodes. Electrodes and a separatordisposed between two electrodes were then assembled and encased in anAl-plastic laminated packaging envelop, followed by liquid electrolyteinjection to form a sodium or potassium battery cell.

Example 5 Preparation of Pristine Graphene Sheets (0% oxygen)

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to aconductive additive having a high electrical and thermal conductivity.Pre-sodiated pristine graphene was also used as an anode activematerial. Pristine graphene sheets were produced by using the directultrasonication or liquid-phase production process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.Pristine graphene is essentially free from any non-carbon elements.

Pristine graphene sheets, as a conductive additive, along with an anodeactive material (or cathode active material in the cathode) were thenincorporated in a battery using both the presently invented procedure ofslurry injection into foam pores and conventional procedure of slurrycoating, drying and layer laminating. Both alkali metal-ion batteriesand alkali metal batteries (injection into cathode only) wereinvestigated. In the latter batteries, primary or secondary, the anodeis either Na foil or K chips supported by graphene sheets.

Example 6 Preparation of Pre-Sodiated Graphene Fluoride Sheets as anAnode Active Material of a Sodium-Sulfur Battery

Several processes have been used by us to produce graphene fluoride(GF), but only one process is herein described as an example. In atypical procedure, highly exfoliated graphite (HEG) was prepared fromintercalated compound C₂F.xClF₃. HEG was further fluorinated by vaporsof chlorine trifluoride to yield fluorinated highly exfoliated graphite(FHEG). Pre-cooled Teflon reactor was filled with 20-30 mL of liquidpre-cooled ClF₃, the reactor was closed and cooled to liquid nitrogentemperature. Then, no more than 1 g of HEG was put in a container withholes for ClF₃ gas to access and situated inside the reactor. In 7-10days a gray-beige product with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol and ethanol, separately)and subjected to an ultrasound treatment (280 W) for 30 min, leading tothe formation of homogeneous yellowish dispersions. Upon removal ofsolvent, the dispersion became a brownish powder. The graphene fluoridepowder was mixed with sodium chips in a liquid electrolyte, allowing forpre-sodiation to occur before or after injection into pores of an anodecurrent collector.

Example 7 Preparation of Nitrogenataed Graphene Nano Sheets and PorousGraphene Structures

Graphene oxide (GO), synthesized in Example 1, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene:urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenataed graphene sheets remaindispersible in water. Two types of dispersions were then prepared. Oneinvolved adding water-soluble polymer (e.g. polyethylene oxide) into thenitrogenated graphene sheet-water dispersion to produce a water-basedsuspension. The other involved drying the nitrogenated graphenesheet-water dispersion to recover nitrogenated graphene sheets, whichwere then added into precursor polymer-solvent solutions to obtainorganic solvent-based suspensions.

The resulting suspensions were then cast, dried, carbonized andgraphitized to produce porous graphene structures. The carbonizationtemperatures for comparative samples are 900-1,350° C. Thegraphitization temperatures are from 2,200° C. to 2,950° C. The porousgraphene layers are used as the porous current collectors for both theanode and the cathode of Li—S cells.

Example 8 Conductive Web of Filaments From Electro-Spun PAA Fibrils as aSupporting Layer for the Anode

Poly (amic acid) (PAA) precursors for spinning were prepared bycopolymerizing of pyromellitic dianhydride (Aldrich) and4,4′-oxydianiline (Aldrich) in a mixed solvent oftetrahydrofurane/methanol (THF/MeOH, 8/2 by weight). The PAA solutionwas spun into fiber web using an electrostatic spinning apparatus. Theapparatus consisted of a 15 kV d.c. power supply equipped with thepositively charged capillary from which the polymer solution wasextruded, and a negatively charged drum for collecting the fibers.Solvent removal and imidization from PAA were performed concurrently bystepwise heat treatments under air flow at 40° C. for 12 h, 100° C. for1 h, 250° C. for 2 h, and 350° C. for 1 h. The thermally cured polyimide(PI) web samples were carbonized at 1,000° C. to obtain carbonizednano-fibers with an average fibril diameter of 67 nm. Such a web can beused as a conductive substrate for an anode active material. We observethat the implementation of a network of conductive nano-filaments at theanode of a Li—S cell can effectively suppress the initiation and growthof lithium dendrites that otherwise could lead to internal shorting.

Example 9 Electrochemical Deposition of S on Various Webs or PaperStructures (External Electrochemical Deposition) for Li—S and Na—SBatteries

The electrochemical deposition may be conducted before the cathodeactive layer is incorporated into an alkali metal-sulfur battery cell(Li—S or Na—S cell). In this approach, the anode, the electrolyte, andthe integral layer of porous graphene structure (serving as a cathodelayer) are positioned in an external container outside of alithium-sulfur cell. The needed apparatus is similar to anelectro-plating system, which is well-known in the art.

In a typical procedure, a metal polysulfide (M_(x)S_(y)) is dissolved ina solvent (e.g. mixture of DOL/DME in a volume ratio from 1:3 to 3:1) toform an electrolyte solution. An amount of a lithium salt may beoptionally added, but this is not required for external electrochemicaldeposition. A wide variety of solvents can be utilized for this purposeand there is no theoretical limit to what type of solvents can be used;any solvent can be used provided that there is some solubility of themetal polysulfide in this desired solvent. A greater solubility wouldmean a larger amount of sulfur can be derived from the electrolytesolution.

The electrolyte solution is then poured into a chamber or reactor undera dry and controlled atmosphere condition (e.g. He or Nitrogen gas). Ametal foil can be used as the anode and a layer of the porous graphenestructure as the cathode; both being immersed in the electrolytesolution. This configuration constitutes an electrochemical depositionsystem. The step of electrochemically depositing nano-scaled sulfurparticles or coating on the graphene surfaces is conducted at a currentdensity preferably in the range of 1 mA/g to 10 A/g, based on the layerweight of the porous graphene structure.

The chemical reactions that occur in this reactor may be represented bythe following equation: M_(x)S_(y)→M_(x)S_(y-z)+zS (typically z=1-4).Quite surprisingly, the precipitated S is preferentially nucleated andgrown on massive graphene surfaces to form nano-scaled coating or nanoparticles. The coating thickness or particle diameter and the amount ofS coating/particles may be controlled by the specific surface area,electro-chemical reaction current density, temperature and time. Ingeneral, a lower current density and lower reaction temperature lead toa more uniform distribution of S and the reactions are easier tocontrol. A longer reaction time leads to a larger amount of S depositedon graphene surfaces and the reaction is ceased when the sulfur sourceis consumed or when a desired amount of S is deposited. These S-coatedpaper or web structures were then pulverized into fine particles for useas the cathode active material of a Li—S or Na—S cell.

Example 10 Chemical Reaction-Induced Deposition of Sulfur Particles onIsolated Graphene Sheets Prior to Cathode Layer Preparation

A prior art chemical deposition method is herein utilized to prepareS-graphene composites from isolated graphene oxide sheets (i.e. these GOsheets were not packed into an integral structure of porous grapheneprior to chemical deposition of S on surfaces of GO sheets). Theprocedure began with adding 0.58 g Na₂S into a flask that had beenfilled with 25 ml distilled water to form a Na₂S solution. Then, 0.72 gelemental S was suspended in the Na₂S solution and stirred with amagnetic stirrer for about 2 hours at room temperature. The color of thesolution changed slowly to orange-yellow as the sulfur dissolved. Afterdissolution of the sulfur, a sodium polysulfide (Na₂S_(x)) solution wasobtained (x=4-10).

Subsequently, a graphene oxide-sulfur (GO-S) composite was prepared by achemical deposition method in an aqueous solution. First, 180 mg ofgraphite oxide was suspended in 180 ml ultrapure water and thensonicated at 50° C. for 5 hours to form a stable graphene oxide (GO)dispersion. Subsequently, the Na₂S_(x) solution was added to theabove-prepared GO dispersions in the presence of 5 wt % surfactant cetyltrimethyl-ammonium bromide (CTAB), the as-prepared GO/Na₂S_(x) blendedsolution was sonicated for another 2 hours and then titrated into 100 mlof 2 mol/L HCOOH solution at a rate of 30-40 drops/min and stirred for 2hours. Finally, the precipitate was filtered and washed with acetone anddistilled water several times to eliminate salts and impurities. Afterfiltration, the precipitate was dried at 50° C. in a drying oven for 48hours. The reaction may be represented by the following reaction: S_(x)²⁻+2H⁺→(x-1)S+H₂S.

Example 11 Redox Chemical Reaction-Induced Deposition of SulfurParticles on Isolated Graphene Sheets Prior to Cathode Layer Preparation

In this chemical reaction-based deposition process, sodium thiosulfate(Na₂S₂O₃) was used as a sulfur source and HCl as a reactant. A GO-watersuspension was prepared and then the two reactants (HCl and Na₂S₂O₃)were poured into this suspension. The reaction was allowed to proceed at25-75° C. for 1-3 hours, leading to the precipitation of S particlesdeposited on surfaces of GO sheets. The reaction may be represented bythe following reaction:

2HCl+Na₂S₂O₃→2NaCl+S↓+SO₂↑+H₂O.

Example 12 Preparation of S/GO Nanocomposites Via Solution Deposition

GO sheets and S were mixed and dispersed in a solvent (CS₂) to form asuspension. After thorough stirring, the solvent was evaporated to yielda solid nanocomposite, which was then ground to yield nanocompositepowder. The primary sulfur particles in these nanocomposite particleshave an average diameter of approximately 40-50 nm.

Example 13 Preparation and Electrochemical Testing of Various BatteryCells5

For most of the anode and cathode active materials investigated, weprepared alkali metal-sulfur cells or alkali metal ion-sulfur cellsusing both the presently invented method and the conventional method.

With the conventional method, a typical anode composition includes 85wt. % active material (e.g., Sn- or Na₂C₈H₄O₄-coated graphene sheets forNa ion-sulfur anode; graphite or Si particles for Li ion-sulfur anode),7 wt. % acetylene black (Super-P), and 8 wt. % polyvinylidene fluoridebinder (PVDF, 5 wt. % solid content) dissolved inN-methyl-2-pyrrolidinoe (NMP). After coating the slurries on Cu foil,the electrodes were dried at 120° C. in vacuum for 2 h to remove thesolvent. Cathode layers are made in a similar manner (using Al foil asthe cathode current collector). An anode layer, separator layer (e.g.Celgard 2400 membrane), and a cathode layer are then laminated togetherand housed in a plastic-Al envelop. The cell is then injected with 1 MLiPF₆ or NaPF₆ electrolyte solution dissolved in a mixture of ethylenecarbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v). In somecells, ionic liquids were used as the liquid electrolyte. The cellassemblies were made in an argon-filled glove-box.

In the presently invented process, in certain examples, the anodecurrent collector (conductive porous structure for the anode), theseparator, and the cathode current collector (conductive porousstructure for the cathode side) are assembled in a protective housingbefore or after the injecting (or impregnation) of the first suspensionand/or the injecting (or impregnation) of the second suspension. In someexamples, we assembled an empty foamed anode current collector, a porousseparator layer, and an empty foamed current collector together to forman assembly that was housed in a pouch (made of Al-nylon bi-layer film).The first suspension was then injected into the anode current collectorand the second suspension was injected into the cathode currentcollector. The pouch was then sealed. In other examples, we impregnateda foamed anode current collector with the first suspension to form ananode layer and, separately, impregnated a foamed cathode currentcollector with the second suspension to form a cathode layer. The anodelayer, a porous separator layer, and the cathode layer were thenassembled and housed in a pouch to form a cell. With the instant method,typically no binder resin is needed or used, saving 8% weight (reducedamount of non-active materials).

The cyclic voltammetry (CV) measurements were carried out using an Arbinelectrochemical workstation at a typical scanning rate of 1 mV/s. Inaddition, the electrochemical performances of various cells were alsoevaluated by galvanostatic charge/discharge cycling at a current densityof from 50 mA/g to 10 A/g. For long-term cycling tests, multi-channelbattery testers manufactured by LAND were used.

It may be noted that, in lithium-ion battery industry, it is a commonpractice to define the cycle life of a battery as the number ofcharge-discharge cycles that the battery suffers 20% decay in capacitybased on the initial capacity measured after the requiredelectrochemical formation. The same definition for the cycle life of aLi—S or room temperature Na—S cell is herein followed.

Example 14 Representative Testing Results

For each sample, several current densities (representingcharge/discharge rates) were imposed to determine the electrochemicalresponses, allowing for calculations of energy density and power densityvalues required of the construction of a Ragone plot (power density vs.energy density). Shown in FIG. 5 are Ragone plots (gravimetric andvolumetric power density vs. energy density) of Na-ion battery cellscontaining hard carbon particles as the anode active material andactivated carbon/sulfur composite particles as the cathode activematerials. Two of the 4 data curves are for the cells prepared accordingto an embodiment of instant invention and the other two by theconventional slurry coating of electrodes (roll-coating of slurry).Several significant observations can be made from these data:

Both the gravimetric and volumetric energy densities and power densitiesof the sodium ion-S battery cells prepared by the presently inventedmethod (denoted as “inventive” in the figure legend) are significantlyhigher than those of their counterparts prepared via the conventionalroll-coating method (denoted as “conventional”). A change from an anodethickness of 150 μm (coated on a flat solid Cu foil) to a thickness of225 μm (all accommodated in pores of a Ni foam having 85% porosity) anda corresponding change in the cathode to maintain a balanced capacityratio results in a gravimetric energy density increase from 155 Wh/kg to187 Wh/kg. Even more surprisingly, the volumetric energy density isincreased from 232 Wh/L to 318 Wh/L.

These significant differences cannot be simply ascribed to the increasesin the electrode thickness and the mass loading. The differences arelikely due to the significantly higher active material mass loading(relative to other materials) associated with the presently inventedcells, reduced proportion of overhead (non-active) components relativeto the active material weight/volume, no need to have a binder resin,surprisingly better utilization of the electrode active material (most,if not all, of the hard carbon particles and C/S particles contributingto the sodium ion storage capacity; no dry pockets or ineffective spotsin the electrode, particularly under high charge/discharge rateconditions), and the surprising ability of the inventive method to moreeffectively pack active material particles in the pores of the foamedcurrent collector.

FIG. 6 shows the Ragone plots (both gravimetric and volumetric powerdensity vs. gravimetric and volumetric energy density) of two cells,both containing graphene-embraced Na nano particles as the anode activematerial and S-coated graphene sheets as the cathode active material.The experimental data were obtained from the battery cells that wereprepared by the presently invented method and those by the conventionalslurry coating of electrodes.

These data indicate that both the gravimetric and volumetric energydensities and power densities of the battery cells prepared by thepresently invented method are significantly higher than those of theircounterparts prepared via the conventional method. Again, thedifferences are huge. The conventionally made cells exhibit agravimetric energy density of 215 Wh/kg and volumetric energy density of323 Wh/L, but the presently invented cells deliver 334 Wh/kg and 601Wh/L, respectively. The cell-level volumetric energy density of 601 Wh/Lhas never been previously achieved with any rechargeable sodiumbatteries. The power densities as high as 1432 W/kg and 2,578 W/L arealso unprecedented for typically higher-energy lithium-ion batteries,let alone for sodium-ion batteries.

These energy density and power density differences are mainly due to thehigh active material mass loading (>25 mg/cm² in the anode and >35mg/cm² in the cathode) associated with the presently invented cells,reduced proportion of overhead (non-active) components relative to theactive material weight/volume, no need to have a binder resin, theability of the inventive method to better utilize the active materialparticles (all particles being accessible to liquid electrolyte and fastion and electron kinetics), and to more effectively pack active materialparticles in the pores of the foamed current collectors.

Shown in FIG. 7 are Ragone plots of Li—S batteries containing a lithiumfoil as the anode active material, S-coated graphene sheets as thecathode active material, and lithium salt (LiaPF₆)-PC/DEC as organicliquid electrolyte. The data are for both sodium metal cells prepared bythe presently invented method and those by the conventional slurrycoating of electrodes. These data indicate that both the gravimetric andvolumetric energy densities and power densities of the sodium metalcells prepared by the presently invented method are significantly higherthan those of their counterparts prepared via the conventional method.Again, the differences are huge and are likely due to the significantlyhigher active material mass loading associated with the presentlyinvented cells, reduced proportion of overhead (non-active) componentsrelative to the active material weight/volume, no need to have a binderresin, surprisingly better utilization of the electrode active material(most, if not all, of the active material contributing to the sodium ionstorage capacity; no dry pockets or ineffective spots in the electrode,particularly under high charge/discharge rate conditions), and thesurprising ability of the inventive method to more effectively packactive material particles in the pores of the foamed current collector.

Quite noteworthy and unexpected is the observation that the cell-levelgravimetric energy density of the presently invented Li—S cell is ashigh as 624 Wh/kg, higher than those of all rechargeable lithium metalor lithium-ion batteries ever reported (recall that current Li-ionbatteries typically store 150-250 Wh/kg based on the total cell weightand 500-650 Wh/L per cell volume). Furthermore, for sulfur cathodeactive material-based lithium batteries, a volumetric energy density of1,185 Wh/L, a gravimetric power density of 2,457 W/kg and volumetricpower density of 4,668 W/L would have been un-thinkable.

It is of significance to point out that reporting the energy and powerdensities per weight of active material alone on a Ragone plot, as didby many researchers, may not give a realistic picture of the performanceof the assembled battery cell. The weights of other device componentsalso must be taken into account. These overhead components, includingcurrent collectors, electrolyte, separator, binder, connectors, andpackaging, are non-active materials and do not contribute to the chargestorage amounts. They only add weights and volumes to the device. Hence,it is desirable to reduce the relative proportion of overhead componentweights and increase the active material proportion. However, it has notbeen possible to achieve this objective using conventional batteryproduction processes. The present invention overcomes thislong-standing, most serious problem in the art of lithium batteries.

In commercial lithium-ion batteries having an electrode thickness of 150μm, the weight proportion of the anode active material (e.g. graphite orcarbon) in a lithium-ion battery is typically from 12% to 17%, and thatof the cathode active material (for inorganic material, such as LiMn₂O₄)from 22% to 41%, or from 10% to 15% for organic or polymeric. Thecorresponding weight fractions in Na-ion batteries are expected to bevery similar since both the anode active materials and cathode activematerials have similar physical densities between two types of batteriesand the ratio of cathode specific capacity to the anode specificcapacity is also similar. Hence, a factor of 3 to 4 may be used toextrapolate the energy or power densities of the device (cell) from theproperties based on the active material weight alone. In most of thescientific papers, the properties reported are typically based on theactive material weight alone and the electrodes are typically very thin(<<100 μm and mostly <<50 μm). The active material weight is typicallyfrom 5% to 10% of the total device weight, which implies that the actualcell (device) energy or power densities may be obtained by dividing thecorresponding active material weight-based values by a factor of 10 to20. After this factor is taken into account, the properties reported inthese papers do not really look any better than those of commercialbatteries. Thus, one must be very careful when it comes to read andinterpret the performance data of batteries reported in the scientificpapers and patent applications.

FIG. 8 shows the Ragone plot of a series of Li ion-S cells(graphene-wrapped Si nano particles, or pre-lithiated Si nano particles)prepared by the conventional slurry coating process and the Ragone plotof corresponding cells prepared by the presently invented process. Thesedata again demonstrate the effectiveness of the presently inventedprocess in imparting unexpectedly high energy densities, bothgravimetric and volumetric, to the Li—S battery cells.

Example 15 Achievable Electrode Thickness and its Effect onEelectrochemical Performance of Lithium Battery Cells

One might be tempted to think the electrode thickness of an alkali metalbattery is a design parameter that can be freely adjusted foroptimization of device performance. Contrary to this perception, inreality, the alkali metal battery electrode thickness ismanufacturing-limited and one cannot produce electrodes of goodstructural integrity that exceed certain thickness level in a realindustrial manufacturing environment (e.g. a roll-to-roll coatingfacility). The conventional battery electrode design is based on coatingan electrode layer on a flat metal current collector, which has severalmajor problems: (a) A thick coating on Cu foil or Al foil requires along drying time (requiring a heating zone 30-100 meters long). (b)Thick electrodes tend to get delaminated or cracked upon drying andsubsequent handling, and even with a resin binder proportion as high as15-20% to hopefully improve the electrode integrity this problem remainsa major limiting factor. Thus, such an industry practice of roll-coatingof slurry on a solid flat current collector does not allow for highactive material mass loadings. (c) A thick electrode prepared bycoating, drying, and compression makes it difficult for electrolyte(injected into a cell after the cell is made) to permeate through theelectrode and, as such, a thick electrode would mean many dry pockets orspots that are not wetted by the electrolyte. This would imply a poorutilization of the active materials. The instant invention solves theselong-standing, critically important issues associated with alkali metalbatteries.

Shown in FIG. 9 are the cell-level gravimetric (Wh/kg) and volumetricenergy densities (Wh/L) of Li ion-S cells (Pre-lithiated graphiteanode+RGO-supported S cathode) plotted over the achievable cathodethickness range of the S/RGO cathode prepared via the conventionalmethod without delamination and cracking and those by the presentlyinvented method.

The electrodes can be fabricated up to a thickness of 100-200 μm usingthe conventional slurry coating process. However, in contrast, there isno theoretical limit on the electrode thickness that can be achievedwith the presently invented method. Typically, the practical electrodethickness is from 10 μm to 1000 μm, more typically from 100 μm to 800μm, and most typically from 200 μm to 600 μm.

These data further confirm the surprising effectiveness of the presentlyinvented method in producing ultra-thick lithium or sodium batteryelectrodes not previously achievable. These ultra-thick electrodes insodium metal batteries lead to exceptionally high sulfur cathode activematerial mass loading, typically significantly >15 mg/cm² (moretypically >20 mg/cm², further typically >30 mg/cm², often >40 mg/cm²,and even >50 mg/cm²). These high active material mass loadings have notbeen possible to obtain with conventional alkali metal-sulfur batteriesmade by the slurry coating processes. These high active material massloadings led to exceptionally high gravimetric and volumetric energydensities that otherwise have not been previously achieved given thesame battery system.

Dendrite issues commonly associated with Li, Na, and K metal secondarycells are also resolved by using the presently invented foamed currentcollector strategy. Hundreds of cells have been investigated and thosecells having a foamed anode current collector were not found to fail dueto dendrite penetration through the separator. SEM examination ofsamples from presently invented sodium and potassium cells confirms thatthe re-deposited alkali metal surfaces on pore walls in a porous anodecurrent collector appear to be smooth and uniform, exhibiting no sign ofsharp metal deposit or tree-like features as often observed withcorresponding cells having a solid current collector (Cu foil) at theanode. This might be due to a reduced exchange current densityassociated with a high specific surface area of the foamed currentcollector at the anode and a more uniform local electric field in such afoamed structure that drives the alkali metal deposition during repeatedre-charge procedures.

1. A process for producing an alkali metal-sulfur battery, wherein saidalkali metal is selected from lithium (Li) and/or sodium (Na), saidprocess comprising: (A) Assembling a porous cell framework composed of afirst conductive porous structure as a cathode current collector, ananode current collector, and a porous separator disposed between saidanode and cathode current collectors; wherein said first conductiveporous structure has a thickness no less than 200 μm and at least 70% byvolume of pores and said anode current collector has two opposed primarysurfaces and at least one of the two primary surfaces contains a layerof sodium or lithium metal or alloy having at least 50% by weight ofsodium or lithium element in said alloy; (B) Preparing a firstsuspension of a cathode active material dispersed in a first liquidelectrolyte, wherein said cathode active material is selected fromsulfur, lithium polysulfide, sodium polysulfide, sulfur-polymercomposite, organo-sulfide, sulfur-carbon composite, sulfur-graphenecomposite, or a combination thereof; and (C) Injecting said firstsuspension into pores of said first conductive porous structure to forma cathode electrode to an extent that said cathode active materialconstitutes an electrode active material loading no less than 7 mg/cm²,and wherein said anode, said separator, and said cathode are assembledin a protective housing before or after said injecting step isconducted.
 2. The process of claim 1, wherein said cathode activematerial is selected from sulfur bonded to pore walls of said cathodecurrent collector, sulfur bonded to or confined by a carbon or graphitematerial, sulfur bonded to or confined by a polymer, sulfur-carboncompound, metal sulfide M_(x)S_(y), wherein x is an integer from 1 to 3and y is an integer from 1 to 10, and M is a metal element selected fromLi, Na, K, Mg, Ca, a transition metal, a metal from groups 13 to 17 ofthe periodic table, or a combination thereof.
 3. The process of claim 1,wherein said conductive porous structure comprises metal foam, metal webor screen, perforated metal sheet-based structure, metal fiber mat,metal nanowire mat, conductive polymer nano-fiber mat, conductivepolymer foam, conductive polymer-coated fiber foam, carbon foam,graphite foam, carbon aerogel, carbon xerox gel, graphene foam, grapheneoxide foam, reduced graphene oxide foam, carbon fiber foam, graphitefiber foam, exfoliated graphite foam, or a combination thereof.
 4. Theprocess of claim 1, wherein a cathode thickness-to-cathode currentcollector thickness ratio is from 0.8/1 to 1/0.8, and/or said cathodeactive material constitutes an electrode active material loading greaterthan 15 mg/cm², and said cathode current collector has a thickness noless than 300 μm.
 5. The process of claim 1, wherein said cathode activematerial is supported by a functional material or nano-structuredmaterial selected from the group consisting of: (A) A nano-structured orporous disordered carbon material selected from particles of a softcarbon, hard carbon, polymeric carbon or carbonized resin, meso-phasecarbon, coke, carbonized pitch, carbon black, activated carbon,nano-cellular carbon foam or partially graphitized carbon; (B) A nanographene platelet selected from a single-layer graphene sheet ormulti-layer graphene platelet; (C) A carbon nanotube selected from asingle-walled carbon nanotube or multi-walled carbon nanotube; (D) Acarbon nano-fiber, nano-wire, metal oxide nano-wire or fiber, conductivepolymer nano-fiber, or a combination thereof; (E) A carbonyl-containingorganic or polymeric molecule; (F) A functional material containing acarbonyl, carboxylic, or amine group to reversibly capture sulfur; andcombinations thereof.
 6. The process of claim 1, wherein said anodecontains an alkali ion source selected from an alkali metal, an alkalimetal alloy, a mixture of alkali metal or alkali metal alloy with analkali intercalation compound, an alkali element-containing compound, ora combination thereof.
 7. The process of claim 1, wherein said anodecontains an alkali intercalation compound selected from petroleum coke,carbon black, amorphous carbon, activated carbon, hard carbon, softcarbon, templated carbon, hollow carbon nanowires, hollow carbon sphere,natural graphite, artificial graphite, lithium or sodium titanate,NaTi₂(PO₄)₃, Na₂Ti₃O₇, Na₂C₈H₄O₄, Na₂TP, Na_(x)TiO₂ (x=0.2 to 1.0),Na₂C₈H₄O₄, carboxylate based materials, C₈H₄Na₂O₄, C₈H₆O₄, C₈H₅NaO₄,C₈Na₂F₄O₄,C₁₀H₂Na₄O₈, C₁₄H₄O₆, C₁₄H₄Na₄O₈, or a combination thereof. 8.The process of claim 1, wherein said first liquid electrolyte isselected from aqueous electrolyte, an organic electrolyte, ionic liquidelectrolyte, mixture of an organic electrolyte and an ionic electrolyte,or a mixture thereof with a polymer.
 9. The process of claim 8, whereinsaid aqueous electrolyte contains a sodium salt or a lithium saltdissolved in water or a mixture of water and alcohol.
 10. The process ofclaim 9, wherein said sodium salt or lithium salt is selected fromNa₂SO₄, Li₂SO₄, a mixture thereof, NaOH, LiOH, NaCl, LiCl, NaF, LiF,NaBr, LiBr, NaI, LiI, or a mixture thereof.
 11. The process of claim 8,wherein said organic electrolyte contains a liquid organic solventselected from the group consisting of 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofloroether, and combinationsthereof.
 12. The process of claim 8, wherein said organic electrolytecontains an alkali metal salt selected from lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂, Lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), Lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-metasulfonate (NaCF₃SO₃), potassium trifluoro-metasulfonate(KCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂),sodium trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethylsulfonylimide potassium (KN(CF₃SO₂)₂), or a combination thereof.
 13. Theprocess of claim 8, wherein said ionic liquid electrolyte contains anionic liquid solvent selected from a room temperature ionic liquidhaving a cation selected from tetra-alkylammonium, di-, tri-, ortetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, trialkylsulfonium, or acombination thereof.
 14. The process of claim 13, wherein said ionicliquid solvent is selected from a room temperature ionic liquid havingan anion selected from BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻,C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻,N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻,SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, or a combination thereof.
 15. Aprocess for producing an alkali metal-sulfur battery, wherein saidalkali metal is selected from lithium (Li) and/or sodium (Na), saidprocess comprising: (A) Preparing at least one or a plurality ofelectrically conductive porous structures, and one or a plurality of wetcathode layers of a cathode active material with a liquid electrolyte,wherein said cathode active material is selected from sulfur, lithiumpolysulfide, sodium polysulfide, sulfur-polymer composite,organo-sulfide, sulfur-carbon composite, sulfur-graphene composite, or acombination thereof, and wherein said conductive porous layers containinterconnected conductive pathways and at least 80% by volume of pores;(B) Preparing an anode electrode having an anode current collector thathas two opposed primary surfaces wherein at least one of the two primarysurfaces is deposited with a layer of alkali metal or alkali metal alloyhaving at least 50% by weight of Na and/or Li element in said alloy; (C)Placing a porous separator layer in contact with said anode electrode;(D) Stacking and consolidating a desired number of said porous layersand a desired number of said wet cathode layers in an alternatingsequence to form a cathode electrode in contact with said porousseparator, wherein said cathode electrode has a thickness no less than200 μm; wherein said step (D) is conducted before or after step (B); and(E) Assembling and sealing said anode electrode, porous separator, andcathode electrode in a housing to produce said alkali metal battery;wherein said cathode active material has a material mass loading no lessthan 10 mg/cm² in said cathode electrode.
 16. The Process of claim 15wherein said wet cathode layers of a cathode active material mixed witha liquid electrolyte of step (A) are mixed with a conductive additive.17. The process of claim 15, wherein said cathode active material isselected from sulfur bonded to pore walls of said cathode currentcollector, sulfur bonded to or confined by a carbon or graphitematerial, sulfur bonded to or confined by a polymer, sulfur-carboncompound, metal sulfide M_(x)S_(y), wherein x is an integer from 1 to 3and y is an integer from 1 to 10, and M is a metal element selected fromLi, Na, K, Mg, Ca, a transition metal, a metal from groups 13 to 17 ofthe periodic table, or a combination thereof.
 18. The process of claim15, wherein said conductive porous layers are selected from metal foam,metal web or screen, perforated metal sheet-based structure, metal fibermat, metal nanowire mat, conductive polymer nano-fiber mat, conductivepolymer foam, conductive polymer-coated fiber foam, carbon foam,graphite foam, carbon aerogel, carbon xerox gel, graphene foam, grapheneoxide foam, reduced graphene oxide foam, carbon fiber foam, graphitefiber foam, exfoliated graphite foam, or a combination thereof.
 19. Theprocess of claim 15, wherein a cathode thickness-to-cathode currentcollector thickness ratio is from 0.8/1 to 1/0.8, and/or said cathodeactive material constitutes an electrode active material loading greaterthan 15 mg/cm², and said cathode current collector has a thickness noless than 300 μm.
 20. The process of claim 15, wherein said conductiveporous layers contain interconnected conductive pathways and at least90% by volume of pores